Exchange 2007 – Attachment / Message Size Limitations and Why a 40MB limit really means 30MB
No – you can’t set ONE MAGIC FIELD in Exchange to limit the file size of received and sent message and attachment sizes. Which, at first, may not make sense. However, if you spend a little time reading WHY it is designed this way, it makes sense.
The ExchangePedia Blog has done a perfect job in distilling the settings required to limit file size attachments.
However, one thing to note – after performing some tests it would appear Exchange uses the converted MAPI message size to enforce the file size restrictions. Why does this matter? Because, when a normail Internet/MIME email is received, Exchange will convert the email to a MAPI message. Big deal, right? Wrong. MAPI conversion can add up to 30% overhead to the message size.
Soooo, if I send an G-mail with a 22MB attachment, Exchange will convert the received message to MAPI, inflating the size to 32MB!! Once more, Exchange enforces the restrictions not against the original size (22MB) but the converted MAPI message (32MB)!!! End result – although the Exchange server is configured to receive 30MB, the best it can do is 18-20MB. #FAIL!
I’m hope to do some further research, because, as ExchangePedia says in their article, Exchange is supposed to add headers to preserve and enforce using the original email size. I’m almost sure I’m probably wrong – I have to be!!!