When connected to the internet with a Mobile Broadband service, it will not usually be possible to send mail using your normal ISP's SMTP (outgoing) mail server, unless it supports "SMTP Authentication".

Therefore, the email program's settings need to be changed to use the 3G network's own SMTP server and these should be the current server addresses for the various networks:

Vodafone 3G SMTP Servers

smtp.vodafone.net
send.vodafone.net
smtp.vizzavi.co.uk

Three 3G SMTP Servers

smtp-mbb.three.co.uk
smtp.three.co.uk

T-Mobile 3G SMTP Servers

smtp.t-email.co.uk (May need to use SMTP Auth and tick "Use In mail settings")

Orange 3G SMTP Servers

smtp.orange.net
smtp.orange.co.uk

O2 SMTP Servers

smtp.o2.co.uk - GPRS and 3G

Virgin SMTP Servers

mail.virgin.net
smtp.virgin.net

Mike, would it be an idea to include on here the link to the Mobile BB dongle thread that you posted a couple of days ago?