I had the same problem. Google search for the problem yielded me this result:
http://www.tracyandmatt.co.uk/blogs/...obile-smtp-fix
Basically, a registry hack (from XDA-Developers) to circumvent the problem. The link also contains .CAB files to do the registry hack if you are unclear about it.
Here's a direct link to the .CAB file:
http://www.tracyandmatt.co.uk/TnMfix_PPC.CAB
I tried it on my HTC Touch PRO, and now I can send e-mails from my gmail account. (May need soft reset after applying registry fix before it works.)
Update: Well... the Gmail problem happened again despite the registry hack. I might have to try MaryGladys' trick.