I've noticed some emails are bounced although they were delivered.
The bounce email says:
Could not deliver message to the following recipient(s):
Failed Recipient: xxxxx@xxxxxxx.xx
Reason: Remote host said: 601 Attempted to send the message to the following ip's:
xxx.xxx.xxx.xxx
The ip is correct for the receiving mail server.
I'm not sure if there is a connection but this happened when the email had a pdf or docx file attached. If I send a plain email there is no bounce.
If I send from another mailserver (Gmail) there is no bounce even with file attached.
They receive emails from different mail servers also with attachment all the time and no one else have complained about bounces.
In delivery log I can see all 4 attempts to deliver until it bounces although the mail was delivered at first try.
Can anyone explain this?