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Exchange Server SE (Subscription Edition) and POP3/IMAP mailbox retrieval: what changes, what does not

Exchange Server SE keeps the Exchange 2019 mail-flow model — receive connectors, send connectors, accepted domains — so a POP3/IMAP connector that submits its mail over SMTP works as it did before, and nothing in SE adds a built-in POP3 connector. Microsoft describes the first release, Exchange SE RTM, as “code equivalent to Exchange Server 2019 CU15” with three exceptions: the licence agreement, the product name and the build number. What does change for a shop that collects provider mailboxes is the route to SE: an in-place upgrade exists only from Exchange 2019 CU14 or CU15, every older version means a new server, and a new server means a new receive connector for the connector to deliver to. This guide lists what Microsoft states about SE, what that means for the connector, and what to check on each upgrade path.

Updated on 2026-10-01

What Exchange Server SE is

Exchange Server Subscription Edition (SE) is the on-premises Exchange version that follows Exchange Server 2019. Microsoft’s lifecycle page lists it under the Modern Lifecycle Policy with a start date of July 1, 2025; the Exchange Server supportability matrix states that “Exchange Server 2016 and Exchange Server 2019 have reached end of support on October 14, 2025”. The page What’s new in Exchange Server SE says about the first release:

• “Exchange Server Subscription Edition (SE) Release to Manufacturing (RTM) is code equivalent to Exchange Server 2019 CU15, except for the following changes”: the licence agreement, the product name, and the build number.
• “New features are introduced starting with the Exchange Server SE Cumulative Update (CU) 1.”
• “You can perform an in-place upgrade from Exchange Server 2019 CU14 or CU15 directly to Exchange Server Subscription Edition (SE).”

The companion page What’s discontinued in Exchange Server repeats the code-equivalence sentence for the step from 2019 to SE and names one removed feature for those coming from Exchange 2016 or 2013, Unified Messaging. Neither page lists a change to receive connectors, send connectors or accepted domains.

What changes and what does not, for a shop that collects POP3/IMAP mailboxes

Topic	What Microsoft states for Exchange SE	What it means for the connector
Fetching from external POP3/IMAP mailboxes	POP3 and IMAP4 exist as client access protocols: “to support clients that still rely on these protocols, you need to start the services” (POP3 and IMAP4 in Exchange Server, applies to 2016, 2019 and SE)	No change: Exchange still does not retrieve mail from provider mailboxes. A separate connector service does that.
Receive connectors	The same documentation page covers 2016, 2019 and SE. The default connector Default Frontend <ServerName> “accepts anonymous connections from external SMTP servers” (Receive connectors in Exchange Server)	No change in how the connector delivers: SMTP to port 25, Anonymous users permission group on the receive connector, as in the Exchange configuration guide.
Where a receive connector lives	“A Receive connector is associated with the Mailbox server or Edge Transport server where it’s created” and is stored in Active Directory as a child object of that server	In-place upgrade: same server, same connector. New server: its own receive connectors, so the permission has to be set there again.
Accepted domains	“Accepted domains are a global setting for the Exchange organization” (Accepted domains in Exchange Server)	They are already there when a new server joins the organization. Verify, do not re-create.
Send connectors	A send connector has source servers, “the Exchange servers where the Send connector is hosted” (Send connectors in Exchange Server)	Matters for outbound relays such as MultiSendcon when the server changes: see below.
TLS defaults	“TLS 1.2 and TLS 1.3 are the only versions which are enabled by default”; “TLS 1.3 is not yet supported for the Simple Mail Transfer Protocol (SMTP)” (What’s new, section for upgrades from Exchange 2016)	Only relevant if Allow StartTLS is switched on in the connector’s Exchange settings. POPcon lists TLS 1.2 / 1.3 encryption; its version history names TLS 1.2 support with version 4.0, so use a current version.
Operating system	Exchange SE runs on Windows Server 2025, 2022 and 2019; Server Core is the recommended installation option (supportability matrix)	The POPcon requirements list the same three Windows Server versions, but do not mention Server Core. See “On the SE server or beside it” below.
Upgrade path	In place from 2019 CU14 or CU15 only; “in-place upgrades from versions of Exchange Server earlier than Exchange Server 2019 are not supported”	From 2016 or 2013 the connector gets a new delivery target.

Why an SMTP-based connector does not depend on the Exchange version

A POP3/IMAP connector such as POPcon is a Windows service that logs in to each provider mailbox, downloads the messages and hands them to Exchange the way any mail server would: over SMTP, to a receive connector. The knowledge base article Does POPcon work with Exchange 2010, 2007, 2003, 2000, 5.5, SBS editions? puts it in one sentence: “POPcon does not use Exchange-specific APIs — it communicates via SMTP, so version upgrades do not affect POPcon’s compatibility.” The POPcon system requirements list Exchange Server SE first among the compatible mail servers.

On the connector side, the complete description of the Exchange server consists of three settings on the Exchange Server page: the server’s hostname or IP address, the SMTP port (25 by default), and Allow StartTLS. None of them names a version. On the Exchange side the counterpart is the receive connector that listens on port 25 with the Anonymous users permission group, plus the accepted domain for your mail domain; both are described for Exchange 2016, 2019 and SE on the same Microsoft pages, linked in the table above. Why Exchange has had no connector of its own since the Small Business Server editions is the subject of Does Exchange Server have a built-in POP3 connector?

The upgrade paths, and what to do with the connector on each

Microsoft’s page distinguishes three starting points. The connector work follows from whether the server stays the same.

Coming from	Microsoft’s path to SE	POP3/IMAP connector
Exchange 2019 CU14 or CU15	In-place upgrade directly to Exchange SE	Same server, same receive connector, same address in the connector. Nothing to reconfigure; run the three checks below.
Exchange 2016	No in-place upgrade. Legacy upgrade to Exchange 2019 CU14 or CU15 first, or a legacy upgrade directly to Exchange SE. Coexistence of 2016 with a 2019/SE organization is supported with Exchange 2016 CU23 on all 2016 servers (system requirements)	New server: set up its receive connector, then change the Exchange server address in the connector. Move the connector first if it runs on the old Exchange server.
Exchange 2013	“You must first perform a legacy upgrade to Exchange Server 2019 CU14 and fully remove Exchange Server 2013 from your organization. Only then can you proceed with an in-place upgrade”	The connector moves once, at the step to the 2019 server; the later in-place upgrade to SE leaves it alone.

After an in-place upgrade (2019 to SE): three checks

1. The Microsoft Exchange Transport service is started on the server.
2. The receive connector on port 25 still carries the Anonymous users permission group (EAC → Mail flow → Receive connectors → Default Frontend → Security).
3. Test Settings on POPcon’s Exchange Server page connects and gets an answer.

If Exchange is not reachable while setup runs, the connector does not lose anything: according to the knowledge base article Will I lose emails if Exchange is down when POPcon tries to deliver?, POPcon keeps every message as a .msg file until Exchange has acknowledged it and retries on every retrieval cycle. If you stop the POPcon service for the duration of the upgrade instead, the mail simply waits in the provider mailboxes.

After a legacy upgrade (new server): the order of work

1. Accepted domains. They are a global setting of the organization, so the new server already knows your mail domain. Confirm it under Mail flow → Accepted domains.
2. Receive connector on the new server. Receive connectors belong to the server they were created on. Enable Anonymous users on the new server’s Default Frontend connector as described in the configuration guide; the steps and the reasoning are in How to download POP3 and IMAP mailboxes into Exchange 2016, 2019 and SE. POPcon always delivers without SMTP authentication.
3. Connector location. If POPcon runs on the old Exchange server, move it before that server is removed: install POPcon on the target machine, stop the service, copy popcon.config and all UIDS_*.dat files, start the service (How can I transfer my POPcon configuration settings to a new server?). The UIDS files are the record of what has already been downloaded.
4. Exchange server address. Enter the new server’s name or IP address in POPcon and use Test Settings.
5. Watch the first cycle. If Exchange answers with an error, the reply code tells you which of the steps above is missing: 550 5.7.1 Unable to relay points at the accepted domain or the receive connector permissions, and the other codes are in Exchange SMTP error codes explained.

Outbound: MultiSendcon and the send connector

MultiSendcon sits on the other side of Exchange: it is an SMTP relay that Exchange reaches through one send connector named “MultiSendcon” on port 2500, and it then picks the provider smart host by sender address. The product page lists Exchange Server SE as supported and notes that it “does not require Exchange agents or transport pipeline plugins”. After an in-place upgrade from 2019 the send connector is still the same object; check with Get-SendConnector that it is enabled and that its address space still carries the cost you set.

With a new server, two properties of that send connector need a look. Its source servers — in Microsoft’s words “the Exchange servers where the Send connector is hosted” — must include the new server before the old one is removed. And its smart host address must be the IP address the MultiSendcon service listens on: the installation guide has you replace the default 127.0.0.1 with the address chosen on MultiSendcon’s Master relay tab, and 127.0.0.1 can only work while MultiSendcon runs on the same machine as the source server. If the connector has to be created again, the knowledge base has the command to create it manually. How Exchange chooses between send connectors is explained in Multiple smart hosts in Exchange 2016/2019.

ChangeSender, the Outlook add-in for choosing the sender address, lists Exchange 2000 through 2019 and Exchange Server SE on its product page.

On the SE server or beside it

Exchange SE is supported on Windows Server 2025, 2022 and 2019, and the POPcon requirements list all three, so the connector can run on the SE server itself. Two points speak for a separate machine when you are setting up a new server anyway. First, Microsoft marks Server Core as the recommended installation option for Exchange SE; the POPcon requirements name the Windows Server versions but say nothing about Server Core, so on a Core installation the member-server placement avoids the question. Second, a connector on its own machine keeps running through every Exchange cumulative update and holds the downloaded messages until Exchange is back. The trade-offs of the three placements are set out in Where to install a POP3/IMAP connector.

If the move to SE is also the moment to reconsider the design as a whole — keep collecting from provider mailboxes, or point the MX record at the new server — the comparison is in POP3 retrieval or MX record to Exchange.

Frequently asked questions

More in the Exchange mail flow guides, on the POPcon product page, the POPcon download page, the MultiSendcon download page or in the knowledge base. Auf Deutsch: Exchange Server SE (Subscription Edition) und POP3-/IMAP-Abholung: was sich ändert — und was nicht