This type of mail server to mail server SMTP failure is usually accompanied by log message like the following:
“SMTP timeout while connected to xx.xx.xx.xx [xx.xx.xx.xx] after sending data block (8189 bytes written): Connection timed out”
This type of failure could arise from any number of network connection issues, firewall packet inspection, a security application, or a virus scanner on the local mail server. However, in every case we have encountered this it has been due to an issue with security systems on the receiving system, typically on the router itself. For the most part, this failure only occurs when the recipient’s scanning system is processing emails with attachments (regardless of size) and it reaches a point where it fails/freezes. After that, the recipient system stops accepting new SMTP data and after a predetermined time, the SMTP connection then times out.