[imp] The message was rejected by the server

[Simon B]

Hi

I got a new iPhone 5s running iOS 8.1.3

ActiveSync is working. Mails arrive and disappear as you'd expect.
Calendar, and contacts too..

The only thing not working is I can't send mails.  I get the error: The
message was rejected by the server.

It's working from my Android device.

SSL is turned on (and worked previously on my iPhone 4.  The only thing I
can think of is that the iPhone doesn't like the server's self-signed
certificate (but that's not what the error is saying...

Any ideas? Known issue?

My install is updated from git every Sunday (the issue occurred last week,
but it was also the first time I had tried to send a mail).

The other exchange account works fine.

Thanks.

Simon