For a while now I have been unable to send my personal email from my mobile phone (running WM6 on the Orange network in the UK). I could download email via IMAP from my server (not hosted by Orange), but never send.
So after doing a bit of research I discoved that the SMTP server is smtp.orange.net, but that did not work. Further playing around with the settings on my found found me clicking the Advanced link under the SMTP server field in the account options settings. Here I choose Orange World as the network, and emails were instantly sent.
Instructions: From your IMAP account in WM6 click Menu > Tools > Options. Click your IMAP account in the list (or add a new account), click Next (filling in fields correctly if adding a new account) until you get to the Outgoing (SMTP) mail server page. Here enter smtp.orange.net and click Advanced Server Settings.
Under Advanced Server Settings select Orange World for the value of the Network Connection. Click Done and then Next a few times until Finish.