This is a new problem. I have Postfix working fine -- I can send email from the mail command in the shell and php mail as well and I receive it just fine.
However, if WordPress sends an email, Postfix can't send, because AWS SES (Amazon SMTP) rejects it with 554 Transaction failed: Duplicate header 'MIME-Version'. I've tried using the "check email" plugin to force send emails from WordPress. Details from the mail log:
Jan 23 08:13:04 webg1 postfix/smtp: E494C7FAC2: to=<firstname.lastname@example.org>, relay=email-smtp.us-west-2.amazonaws.com[184.108.40.206]:587, delay=1, delays=0.02/0.02/0.67/0.31, dsn=5.0.0, status=bounced (host email-smtp.us-west-2.amazonaws.com[220.127.116.11] said: 554 Transaction failed: Duplicate header 'MIME-Version'. (in reply to end of DATA command))
I've learned that AWS SES rejects multiple mime header lines, and it appears WordPress is adding it twice somehow. I also now realize that none of my sites are able to send email (I hadn't noticed it yet).
I will just say one thing:
I'm pretty sure you are having issues with your own configuration. Please, check very carefuly what you are doing.
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