Several months ago (probably a year) my email server (Mercury/32 v4.74) started delivering duplicate messages to local users, but that applies only to messages from some particular domains. Actually it started duplicating emails sent form some companies and with other companies everything works fine. So, if a message is sent from such company to our users "a" and "b", containg field "To: a@pom.com; b@pom.com", then both users "a" and "b" would receive two identical emails (two for each).
After checking email headers (source) I found the following scenario:
- It is actually the same email (ID and originating timestamp) with field "To:" containing both "a@pom.com; b@pom.com".
- At some point while transported betwen different servers email is duplicated (from now on two instances flow in paralel) and different field "Envelope To:" is added to each of them. So, one email with "Envelope To: a@pom.com" and the other with "Envelope To: b@pom.com".
- Both emails are routed correctly to our external domain mailbox and then downloaded by our local Mercury server as two different messages.
- Then, MercuryD delivers BOTH messages to local users "a" and "b". So we have 2 messages for each user, 4 emails in total. And we should have one message for each user.
Somewhere I red that Mercury assumes that "Envelope-To:" fields can be deleted during the transport between servers and that therefore Mercury does not check these fields. Supposingly it checks "To:" and "Bcc:" fields instead, to be sure about the final recepients. That would explain why an email message with a clear "Envelope-To: a@pom.com" is still delivered to both "a@pom.com" and to "b@pom.com".
This wasn't happening in the past so I believe that recently somebody updated some widely used email-server softvare so therefore we got this new behaviour - email clones being sent separately for each recipient. (Two examples would be "smtp.outgoing.loopia.se [194.9.95.112]" and "vimdzpsp-hrel04.bluewin.ch [195.186.101.109]".)
This policy doesn't sound bad as soon as our email servers do read "Envelope-To:" fields. However, it seems that Mercury doesn't do that - and that's why we have a problem. That's what I think. If my conclusions are wrong, please somebody correct me.
So, does anybody have a fix for that? Can we make Mercury read (an use) "Envelope-To:" fields - when these exists?
A good example: no problem happens when an email is sent from Gmail with
"To: a@pom.com; b@pom.com" because Gmail servers put "Envelope To:
a@pom.com; b@pom.com" in one email and keeps transporting only one email
message till the end.
Thanks,
Nenad