Two email accounts which are hosted on an El Capitan mail server are both receiving "duplicate" spam messages. One account reads with Outlook, the other with Mail.app. We started getting the duplicates after moving the email accounts from an EIMS server to El Capitan OS X Server. My accounts on four other servers don't receive duplicate spams.
The message content is identical, messages display identically in both Outlook and Mail.app. Only the headers vary, hence the duplicates are discrete messages. The messages are always in pairs, never threes or more. The duplicates always arrive no more than two minutes after the original.
Spammers will send duplicate messages to bypass greylisting. I don't think this is the cause because other accounts receive spams without duplicates.
Since this started after moving to El Capitan, postfix/dovecot are the most likely suspects.
Ideas?
TL;DR
Our mail server is El Capitan OS X Server with all the latest updates. The Server also does Calendar, File Sharing, Websites, Wiki, and Open Directory.
The Mail server has three domains. The domain receiving the duplicates recently replaced an EIMS server that had been running since 1998. While using EIMS we did did not get duplicate spams. The other two domains are new and don't get a significant amount of spam (yet...).
Mail Filtering has Virus filtering enabled, Blacklist, Greylist, and Junk mail filtering are disabled. We prefer SpamSieve over server-side filtering.
The order of the headers varies. These headers are identical: Return-Path, Delivered-To, Subject, To, From, Content-Transfer-Encoding, and Mime-Version. These headers are identical except for timestamps and ESMTP ids: From and the multiple Received from.
