Outlooks Mail Merge causing recipients to be spammed with attachments

Question

Hi,

A client of mine has been having an odd issue with Outlook recently and we have not been able to narrow down the cause. They use modern Outlook, fully updated, inside of an M365 Entra tenant. They occasionally send out mass emails to over 200 recipients. To do this they use a template, and the native Outlook mail merge. They draft the email as normal, hit send, and everything goes out fine. But they've been getting clients replying asking why they attached the logo.png in their signature dozens of times. We've seen what the recipients are getting and it's a singular email with likely as many logos as emails were sent (we have not counted).

We've tried removing the signature, but this is not a preferred fix. We've also tried embedding/attaching it in various ways and this does nothing. I've read some hypotheses stating it could be an issue with the mail server adding the logo over and over again, or some type of bug, but I'm left leaning towards this simply being an issue with the way Microsoft handles mass emails.

Answer 1

Hi @Anthony Landati

I would not assume this is a mail-flow/server issue yet. The fact that the number of duplicated logo attachments appears to correlate with the number of messages generated by the Mail Merge is a particularly interesting clue.

Microsoft currently lists Mail Merge in New Outlook as only "Partially Available", while it is fully available in Classic Outlook. That makes the New Outlook Mail Merge implementation itself worth isolating before investigating Exchange Online transport.

An image in an HTML signature is normally included as inline MIME content and referenced from the HTML body. If the Mail Merge process is incorrectly reusing or accumulating those MIME parts while generating individual messages, it could potentially explain why recipients receive one message containing many copies of the same logo.png.

I would run a controlled test with perhaps 5 recipients:

New Outlook + Mail Merge + signature logo

New Outlook + Mail Merge without the logo

Classic Outlook + the same Mail Merge + the same signature/logo

Then compare the received messages.

I would also check Message Trace in Exchange Online. If Exchange shows that separate messages were submitted normally to each recipient, but one of the received messages contains multiple copies of the same inline image, I would inspect the message headers/MIME structure rather than assuming Exchange transport duplicated the attachments.

If the problem reproduces only with New Outlook and disappears using Classic Outlook with the same recipients, template and signature, that would strongly suggest a New Outlook/Mail Merge issue.

Since the client is sending to 200+ recipients, I would definitely reproduce this first with a small test group before performing another production mailing.

If possible, please let us know whether the same behavior occurs using Classic Outlook. That comparison would be very useful for narrowing this down.

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