for some reason our e-mails randomly do not get send.

Around 50% of sent e-mail are never received for unknown reason. It seems like in most cases it hits e-mails with an attachment. Those are most likely PGP key files.

We can see the e-mails are in the "sent" folders in the web zoho panel, but the receiver does not get this e-mail. Not in spam, nothing.

Any idea is welcome!

Thanks in advance and best regards,

- Dominik