Need to add the following: 1 - definition of Last External Relay 2 - Information that attachments sent via email are larger than they are on your Hard Drive. if you have an 8MB file and it's in binary format, that's an 8 bit format. but email is a 7 bit format so there is a conversion which increases the size of attachments. I don't think there is an exact percentage because the encoding depends on the exact characters and the type of encoding. However, this article says 30% (http://www.answers.com/topic/binary-and-text-files). So a 7MB attachment could actually be 9.1~ and they might have estimated poorly, etc. Just something to keep in the back of your head. It's the size of the EMAIL not the size of the attachment that matters. And attachments in email are LARGER than they are stored on a computer. KB: (And yes, encoding overhead is rather constant. Basically, there is only base64, encoding 3 byte in exactly 4 byte, which means 33.3% overhead. Plus linebreaks every 76 (encoded) chars, so it's actually more like 35% overhead. Let's ignore the 0-2 padding bytes. ;) 3 - Information about "what is the largest email that can be sent?" Needs to address how email hops from one server to the next and that the size of attachments grows (see #2). And that the size of the email is only as the SMALLEST limit in the chain. For example, here's some notes I wrote to help someone understand their email chain: Outbound Limit: Email goes from your workstation to your Exchange Server (10240 Limit) to the recipient's Server (Unknown Limit) Inbound Limit: Email goes from the sender's server (Unknown Limit) to inbound filtering Servers (30000000 bytes) to your Exchange server (10240 Kilobytes) Internal Limit Email stays on your Exchange server (10240 Kilobytes) In the Inbound/Outbound cases, you'll note that there is a mail server with an unknown limit. Typically, we see limitations of 10MB. And the most important part: The size of an attachment is limited to the the LOWEST limit in the chain of servers. TO COMPLICATE THINGS FURTHER, email is 7-bit (text) and most attachments are 8-bit (binary). The automatic and necessary conversion from 8-bit to 7-bit will increase the size of the attachments. For example, a file that is 3,872,680 bytes in 8-bit is 5,335,721 bytes in 7-bit. This means it's about 38% larger than the original file. This can change based on the content of the file so there is no perfect conversion, I believe. So you need to figure that the limitation above is AFTER the conversion to 7-bit.