I thought I'd post an update on this in case anyone else is having the same problem.
According to various posts on the internet, the HTC mail app doesn't respond to an email intent? Which is very useful:sign0161:
It will however respond to an ActionView intent and using a Uri you can get it to appear in a chooser list and send a text message (no attachments, which is what I was after) with something like this:
UriStr="mailto: ?subject="&SubjectStr&"&body=Test of email intent"
Dim Intent1 As Intent
Intent1.Initialize(Intent1.ACTION_VIEW,UriStr)
StartActivity(Intent1)
To fix my problem I installed K-9 mail, which seems to behave properly and sends attachments which Gmail has a problem with.
I don't really want to have to tell potential users that they have to download another email app to send attachments, but with the current state of Gmail and HTC mail, it may be the only option.