TODO would be nice to implement this, as it is supported on the project server,
but content must be CRLF terminated with a max of 998 chars per line.
And you can't have leading dots either, we'd have to prevent or double-dot it.
That would be expensive to check, using either a double read or
pulling it all into memory.
So it's prohibitive for attachments. We could do it for the message body,
since it's in memory already, but that's not much of a win.