My recepients sometimes receive dupplicate emails
Email duplication results from a problem with mail flow: The mail duplication is due to sender timeout
Issue: When our sending mail servers time out after sending the complete message body, the receiving servers can maintain the SMTP connection. Our server accepts the receiver server’s delivery acknowledgment as an acknowledgment of failure. The sending server retransmits the message.
Resolution: The receiving side’s mail processing server’s need to be upgraded to recognize this behavior as a possible source of duplicate emails, and stop duplicates when the sending server retransmits the message. Most anti-virus and SPAM filtering servers used to exhibit that problem. Companies that use those kinds of servers are encouraged to update their systems or switch to more reliable vendors.