I am running an Ubuntu 18.04.6 LTS server with the postfix 3.1.0-3ubuntu0.4 package.
Both the Apple Mail client and the Postbox client experience a weird delay when first sending an email to the smtp service. Eventually Apple Mail retries and works but Postbox gives up.
In the initial exchange, the email client appears to send a '???' as a command after the server greeting, or at least it is logged that way by postfix. The line in the log looks like (actual ip address redacted):
Nov 23 17:11:09 mail postfix/smtpd: < ipxx-xxx-xxx-xxx.ph.ph.cox.net[xx.xxx.xxx.xxx]: ???
To which the server responds:
502 5.5.2 Error: command not recognized
What is going on here? I cannot find anywhere that the string '???' is considered a valid SMTP command. Is this some sort of hack for older systems?
While both of the above clients seem to send this, the iOS email client does not.
I have not found any documentation of this or tolerating this as a postfix setting.