I have the following setup:
postfix + dovecot-lda
sieve plugin
ldap authentication
dmarc + spf + dkim
say the domain is example.com bob@example.com alice@example.com and charlie@example.com can send emails without any proglems. For a month now however, emails coming from dexter@example.com are marked as spam by gmail, outlook, and many companies. How is it possible that only one of the accounts is marked as spam? I tried different formats (plain, hmtl, rich mails) and attachements (pdfs,images) the result is always the same.
How can I start debugging something like this?