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Why does POPcon need to know the "accepted domains" on the POP3/IMAP config tab?

The need to filter incoming mail so only that sent to one of the accepted domains is due to how POP3 and SMTP mail retrieval/sending works with multiple receivers. This is no problem specific to POPcon, every program retrieving mail by POP3 and forwarding it by SMTP must need to know what domains it is allowed to accept mail for and ignore all other mail.

Consider this example:

Angie at ABC corp. wants to send a mail to both her manager Bob and to you, her client. So she sends the mail:

TO: you@yourdomain.com, bob@abccorp.com

POPcon retrieves the mail and receives no other indication about the intended receiver than what is written in the "TO:" field of the mail.

Unfortunately, the SMTP-protocol needs every single receiver decoded for it and mail server products like Microsoft Exchange™ will complain with a message to the postmaster about every mail they received for another domain. In case of the above mail this would only mean one correctly forwarded message (you@yourdomain.com and one complain-message (bob@abccorp.com: receiver does not exist at this domain) to the postmaster. But factor in rising amount of mails each day and then imagine the reaction to mass-mailings sometimes including dozens of recipients beside the one at your organization. Also, some mail servers will automatically reply to each message with an unknown receiver in it telling the other party the intended receiver does not exist. This would be very strange to Angie: A message from your server, telling her that bob@abccorp is an unknown recipient.

By giving POPcon a list of accepted domains, we enable it to filter only those recipients with specific domain names that EXCHANGE or whatever mail server you use will then accept.