Since upgrading iPhones (4, 4S and 5) to iOS 7 when sending messages that are larger than about 40KB from exchange accounts we are getting the message "Cannot Send Mail. The message was rejected by the server because it is too large."
The same phones don't have any issues sending to from other emails accounts (gmail, yahoo, etc).
We are on Exchange 2010 SP3 RU2.The Exchange server does have size restriction of 50MB.
These accounts have no issue sending the same emails through Outlook or OWA. It is strictly related iOS.
Update to my scenario. I figured it out. We are on Exchange 2007 on Server 2003 and we are using Active Sync with Client Certificates. We did not notice much of a problem when sending using IOS 6 but with IOS 7 it was much more prevalent, any message over 100K would get rejected as too large. turns out it was a problems with IIS 6 and the uploadreadaheadsize value in the IIS metabase. Here is a link regarding it for 2010
here is one for 2007
I bumped mine up to 10Mb and now attachments are going through fine. THis probably should have been done a long time ago, but the symptoms really just reared its ugly head after the ios 7 upgrade.
I hope this helps others, as i was pulling my hair out all weekend trying to figure this out.