Please provide me with access and give me step by step instructions how to reproduce the problem.

Regards,
Daniel

While setting up the necessary accounts for you to access the site, I think I found a setting issue in Mailster that fixes the problem. Will report more after additional testing.

Apologies if this is not relevant, but it might be that we are experiencing a similar issue.

Not all the time, but occasionally emails with attachments being sent via CMandrill are being sent twice.

From the email headers I can see that the first has this line:
X-MS-Exchange-Organization-SenderIdResult: SoftFail

whereas the second has:
X-MS-Exchange-Organization-SenderIdResult: None

Both emails are being received by the recipient.

I have sent a support ticket to Mandrill themselves as well, but just wondering if this is likely to be something to do with CMandrill? (and also whether it was therefore related to the original poster's issue).

Jack, I do not know if they are related or not. I do not have any of the MS-Exchange lines through any of my headers. That said, it does look like the headers may hold some of the keys since they are significantly different; however, I do not have the time to fully parse and sanitize them so that I would feel comfortable posting on an open forum. Also, I cannot tell if it is the Mailster component or the CMandrill component is causing the issue. But regarding the occasional aspect, I have noticed that some file attachments (such as PDFs) do not generate duplicate emails while others (such as CSV and XLSX) will. I have not run through all the various file types to determine which ones may or may not be an issue.

Daniel, I can provide more information on what causes the issue and am ready to grant you access to the site and send detailed instructions on how to replicate. Just let me know how best to send you this information.

Daniel, I have been thinking about what you said on the original post a couple of months back about how if there are attachments that CMandrill falls back on the Joomla mailer. I had been assuming that if/when the Joomla Mailer was used the email would not be sent through Mandrill. However, all of my emails are being formatted with a Mandrill template that I set up specifically for the Mailster mailing lists. I wonder if it might be the fallback process that might be causing the issue. I know that one header has the web address details in the "Received from" header and duplicates have headers as if they were sent through SMTP settings that were in the Joomla config before I installed CMandrill.

Send me a mail to daniel@compojoom.com and I'll try to track it down when I get some time.
Regards,
Daniel

Email with access and replication instructions has been sent.