Microsoft Outlook especially older versions is junk, but you probably knew that already.
For some reason Outlook often sends out a message with incorrect syntax e.g. “[email protected]”.
Mail servers like postfix will reject these messages but Outlook will not warn its user that there is a problem. Instead it will keep the message in the outbox and continously retry sending.
If you are a postfix admin you can strip the apostrophes with a command filter.
Here is how:
Your log will have a message like
warning: Illegal address syntax from server.example.net[192.0.2.20] in RCPT command:<'[email protected]'>
create a file outlookfix.regex and ad the content below:
/^RCPT\s+TO:\s*<'([^[:space:]]+)'>(.*)/ RCPT TO:<$1>$2
Edit /etc/postfix/main.cf and add the line:
smtpd_command_filter = pcre:/etc/postfix/outloofix.regex
Execute postfix reload