Email scanning broken with *.mht attachments

[doktornotor]

1/ When trying to send an email w/ *.mht attachment, Avast just terminates the connection to SMTP server and no mail gets sent
2/ On recieving email w/ *.mht attachment, you get the following message instead of the attachment:

Content type does not match the extension of the included file
* <filename>.mht - message/rfc822

Noted over 2 years ago here, however the "solution" to disable email scanning is really no solution at all. Changing mail shield's heuristics settings did not solve anything either. Using Avast! Free 5.0.594 w/ Outlook 2010.

What's up with this

[doktornotor]

Hmmm, nothing? 

[DavidR]

Well I guess because no one is really familiar with the problem. Not many people are using the .mht format

I tried a test after finding an old .mht file I had saved and got an error with OE6:

An unknown error has occurred. Subject 'Test MHT attached', Account: '0myemail account2', Server: 'smtp.my_email_server.net', Protocol: SMTP, Server Response: '554 Suspicious message', Port: 25, Secure(SSL): No, Server Error: 554, Error Number: 0x800CCC6F

Now the interesting part of the error (which you didn't post in your OP) is the Server Response: '554 Suspicious Message' and I don't know which server this relates to. If it related to the avast Mail Shield acting as a localhost server/proxy then I would have thought that avast would have alerted and that didn't happen.

A very long time ago MS on a security update stupidly started blocking emails in many of their email clients based on nothing more than the email attachment file type, even zip files got blocked. I don't know if this is a throwback to that, but long ago I disabled that pathetic security option, so that shouldn't have been an issue on blocking a file type attachment.

[doktornotor]

Well I guess because no one is really familiar with the problem. Not many people are using the .mht format

I tried a test after finding an old .mht file I had saved and got an error with OE6:

An unknown error has occurred. Subject 'Test MHT attached', Account: '0myemail account2', Server: 'smtp.my_email_server.net', Protocol: SMTP, Server Response: '554 Suspicious message', Port: 25, Secure(SSL): No, Server Error: 554, Error Number: 0x800CCC6F

Now the interesting part of the error (which you didn't post in your OP) is the Server Response: '554 Suspicious Message' and I don't know which server this relates to. If it related to the avast Mail Shield acting as a localhost server/proxy then I would have thought that avast would have alerted and that didn't happen.

Well, I couldn't post any error messages from the mailserver since - for me at least - Avast simply disconnects from the mailserver when trying to send the email w/ *.mht attachment in it, before even trying to send the attachment.

I can live w/o MHT junk, but people occasionally send me such stuff and I need to ask them to send PDF or something else instead. Looks like the mailshield gets completely broken when it hits MHT.

[NON]

I can send / receive mails with mht attachment via my usual mail client "nPOP", but I can't via Windows Live Mail and I got error message that exactly same as DavidR.
I can send / receive without Mail Shield.

It seems this issue depends on mail header form: I can see nPOP encodes mht file into BASE64 format, but WLM don't encode mht file so content of attached mht is contained as it is.
Some small mht file can be sent with Mail Shield enabled via WLM, so it also depends the content of mht files.