I've been having this issue of a new deployment of Exchange 2013 where ActiveSync clients cannot send an attachment of any kind. It immediately fails.
I've found a number of posts regarding upping the send limitations. I've followed all of these to a tooth. These are affecting Outlook and OWA with no problem, but it doesn't matter if the attachment is 100K, ActiveSync clients fail to send the email every time.
The Exchange environment is a single server acting as the CAS, Mailbox, and Transport.
Using Apple Mail on iOS 12.3.1, I attempted to attach a 45.3k image and send it to a gmail account, from an exchange account on the server in question. It resulted in:
Cannot Send Mail
The message was rejected by the server because it is too large.
I confirmed the following settings. They are set to defaults listed below. There is no need to increase these, because as said, ANY attachment, including my test of 45.3KB, fails.
| Client Access | %ExchangeInstallPath%FrontEndHttpProxySyncweb.config | maxRequestLength="10240" | kilobytes |
| Mailbox | %ExchangeInstallPath%ClientAccessSyncweb.config | maxRequestLength="10240" | kilobytes |
| Mailbox | %ExchangeInstallPath%ClientAccessSyncweb.config | <add key="MaxDocumentDataSize" value="10240000"> | bytes |
This error does not happen from Outlook (desktop), OWA, or Outlook mobile app.
I'm at a loss for where to look next but I need to find something else to look at...
Any help is GREATLY appreciated.