We limit our total message size to 50MB.
The e-mail server can handle much larger sizes, but we found that anything more than that and many users would have their mail client lock up on them when it downloaded the message.
Outlook over IMAP always appear to lock up while it downloads the message if you have the preview window enabled. 50MB was a good trade-off between getting support calls for e-mail lockups and support calls for messages bouncing because they are too big.
For larger files, we recommend using DropBox, Cloud.com or any file sharing applications that allow you to upload over 1000MB files and share links for others to download.
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