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In your case it would depend largely on the server, but it sounds like there may be a limit per account or collection of accounts, representing a 'global limit' instead of a per-usage or per-file limit. I'd contact their support department and use usendit.com in the mean time.
I have been trying to send a 15.7 meg servise manual attachment and have been getting the error: you have exceeded the global limit of the server and the message cannot be sent. The manual is a PDF 15.7 meg file but the limit seems to be 22.5 meg. What is and how?