We've successfully migrated from an ancient Microsoft Exchange to dovecot+postfix and everything works fine except from one particular case:
When users of Outlook 2016 hit reply on an old mail from someone within the company (someone@example.com) they get a delivery error from Outlook and sometimes I notice that Outlook doesn't even make a connection to the SMTP server and other times it makes one and gets an error (see below). If they hit forward and type the same address (someone@example.com) manually everything is OK.
The non-delivery-notification email that comes back is in the lines of (translation from Greek):
We couldn't reach one or more recipients...
'Company S.A. John Smith' at 22/10/2018 1:03 pm
Server error: '501 5.1.3 Bad recipient address syntax'"
and postfix has this line in the logs:
postfix/smtps/smtpd[3905]: warning: Illegal address syntax from xxx.yyy.local[192.168.0.153] in RCPT command: <Company S.A. John Smith>
Users that have Thunderbird have no problem at all. Both Outlook and Thunderbird are using the same connection settings for the server (secure IMAP and SMTP)
Any ideas on what goes wrong here would be highly appreciated.