I'm stumped!

When I receive messages with attachments from external mail servers, the attached file size is incorrect. I'm running Exchange 2003 locally. The problem seems to be happening at the SMTP server (the standard MS SMTP server).


I send a 9MB file to a Gmail account. The Exchange message tracking log reports a ~9MB message, but the SMTP server log reports a ~12MB message. Gmail receives the message and reports the attachment correctly as 9MB.

If I forward that same message back to myself from Gmail, the SMTP server reports it coming in as ~12MB, the Exchange log reports it as ~12MB, and in Outlook, the attachment size is reported as 12MB. When I save the attachment, the file on disk is the correct 9MB. No file corruption.

Example 2:

I send a message directly from Gmail to my local account with a file attached.

Original File Size: 9,961,472 bytes
SMTP Server Log: 13,630,172 bytes
Exchange Log: 13,630,168 bytes
Attachment Size Reported By OutlooK: 13MB
Size of file after saving to disk from Outlook: 9,961,472 bytes

As you can see, the file size "changes" somehow when it is received by the SMTP service, but the attached file saves correctly as the original size.

I have an ISA 2004 server in front of Exchange, and SMTP filtering is enabled. I also have ESET NOD32 for Exchange running.

At this point I'm not sure where to look. Has anyone seen this before?


That's to be expected and is perfectly normal behavior. Any non internal, non ASCII email message (including any attachments) needs to be encoded\decoded. Exchange uses Base64 encoding which adds roughly a 30% overhead to the size of the email.


  • I had suspected an encoding issue, thank you. I'll just raise my limits a bit to compensate. – Boden Oct 13 '10 at 23:08
  • Glad to help... – joeqwerty Oct 13 '10 at 23:08

Note that the SMTP log is not telling you how big the Attachment is, it is telling you how big the message is, which includes the Attachment, headers, any text, graphic backgrounds, etc. Now, I'd expect your headers to be less than 10K in size, so the only thing I can think of is if you have an image as your email stationary, or image in a signature, etc that is 3-4MB in size.

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