I have an email setup like this:

Someone sends email to me@mydomain.com - this redirects to myfilter@gmail.com. I use Thunderbird to access myfilter@gmail.com using IMAP and the same goes for my iPhone.

When I send email from Thunderbird I make the sender appear as me@mydomain.com and use my ISP's SMTP server (you have to be on one of the ISP's lines to use it). However, I can't work out how to make email appear to come from me@mydomain.com on my iPhone (2.0, 1st gen, jailbroken & pwned). I am with O2 so I tried putting smtp.o2.co.uk in as the SMTP server but it didn't work (wouldn't connect). My domain provider doesn't have an SMTP server for me to use.

Does anyone know how I can make email from my iPhone appear to come from me@mydomain.com?