All emails are suddenly tagged as SPAM after working fine for a long time

This is usually caused by a DNS configuration problem that makes every antispam blacklist lookup return a positive result.

Diagnosis

Open a command prompt and test a known-clean IP against a blacklist:

ping 2.0.0.127.bl.spamcop.net

This should return IP 127.0.0.2. If it returns anything else, or if the test for a known-bad IP also succeeds, DNS is behaving incorrectly.

Fix

Check your network adapter's TCP/IP DNS settings. Some routers or ISP configurations append domain suffixes to DNS queries, which breaks blacklist lookups. Remove any extra DNS suffix search entries.