Weird behavior of message size limits in Exchange 2007

I've just seen what I think is some bizarre behavior in Exchange 2007 regarding message size limits.  I'm sending from one Exchange 2007 to another Exchange 2007 in different domains.  They are both Exchange 2007 SP3 with the latest rollup installed.  

The sending server has no message size limits.  The receiving server had a 50MB message size limit on the Internet receive connector.  I got a complaint from a user at the receiving end that someone had sent him a message with a 42MB attachment and it was rejected, so I began testing.

First I checked the message size limit on all the receive connectors and found one that had a 20MB limit.  I didn't think this connector should be affecting the Internet receive limits at all, but I changed it anyway.  I tried sending a 43MB attachment, and it was rejected.  Because they use an outside anti-spam/virus filtering service, I checked with them. They did not think their system was affecting this behavior. I had to change some log settings but finally found an SMTP log entry showing that the message was rejected at the receiving server because of the size limit.

Then, I tried resending it one more time and decided to check on the sending server.  In the message queue on the SENDING server, it showed the message size as 60MB. So, I changed the receiving server's limit to 75MB, and sure enough the message went through.

Now the weirdness is that in Outlook on both the sending and receiving end, the message size shows as 43MB.

Anyone have any ideas what this is about?
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Hypercat (Deb)Asked:
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VB ITSConnect With a Mentor Specialist ConsultantCommented:
The size changes due to MIME encoding that gets added onto the message by your server. The general consensus is that the size will grow by roughly 33%.

Email programs (such as Outlook) are aware of this encoding so they hide it.

Have a read of this link if you'd like some more information:
IvanConnect With a Mentor System EngineerCommented:

email with attachment can get you overhead, somewhere from 10-30%, or so. I'm not sure if there is some calculator, but there is overhead. Larger the attachment, larger overhead. I used to have limit of 10mb, but I told users that it is 8mb, so no1 would complain :)

Hypercat (Deb)Author Commented:
I thought it might be something like that, but 17MB is a lot of overhead - closer to 50% than 30% - and obviously more than I was ever expecting to see.  Although some of our clients do deal with large attachments, we always try to discourage it.  I've never heard any statistics about it, though. Where did you get the 10-30% figure?
IvanSystem EngineerCommented:

that is what I have came up with years of exchange administration. When you send email, you can go to EMC, Toolbox, Message Tracking and track some mail you have sent with attachment.

For instance.
I have just send email, with attachment of 10.320.754bytes (9.84MB)
When you look at message tracking, it's size is 14.126.169bytes
That is 36% larger email, because of attachment.

This was just tested on Exchange 2007, that I have. I have never send email larger then 10Mb, but I guess it can go higher then 30%, as just tested.

Hypercat (Deb)Author Commented:
Thanks to you both for your comments.  I've been a system admin for years and simply never had occasion to check message size limits this closely and learn!
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