Email Services - Frequently Asked Questions:
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Do attachments have a size limit?
The maximum message size for Zimbra, Exchange, and Forward-Only is 30MB. However, due to encoding and other factors (like the amount of text in your message), the effective attachment size is smaller. Depending on the type of file you are attaching, the size limit will likely be 20MB - 25MB.
Information Systems and Computing, University of Pennsylvania