Emails Received from Thunderbird on Exchange 2013 CU23 get their attachments corrupted

Copper Contributor

We are facing an issue with emails received from specific people using Thunderbird mail client and sending attachments in languages other than English.

The email is received ok . Content is fine and readable but the attachments are received corrupted with the following type :


"part_of_original_filename Filename2= Filename3=' and the size is a lot smaller for example original doc file 58kb attachment received 200bytes


Any seen this issue before and has any idea on how to resolve it ?

3 Replies



Same issue here, but with Exchange 2016.
Do you happen to have a cloud based antivirus/antispam in front of it?
I can simulate the issue as well, and it only appears when mail is received from thunderbird clients.. And some (external) applications that send SMTP.

The cloud AV provider says the problem is located on Exchange, but we cannot simulate the problem when connecting a thunderbird client internally.

Did you manage to get some more information yourself?

PS : I posted on the mozilla forums as well, but 0,0 respons.. :s

I can reproduce this as soon as Thunderbird is connected by e.g. GMX and the delivery format is set to auto-detect or "plain and Rich (HTML) Text".
If I use "plain Text Only" or "Rich Text (HTML) Only" the attachment is received correctly by Outlook.
We have Exchange 2016 CU19 and Outlook 2016. 

Thanks for the feedback!
I found the issue.. 
The last part of the maiflow wasn't TLS encrypted! Enabling TLS on the receive connector fixed the issue!
Although they told me no network device in between could have modified any packets, and they don't have IPS configured, something was able to change the packets I guess.