I bought my first SmartPhone last week, so my knowledge about them is near zero.
In some cases sending e-mails through mobile internet won't work using the SMTP-settings. In those cases you should use IMAP-settings. Check if that might be the case.
I tried using the IMAP settings, but that didn't make any difference. However, I did try using the K-9 mail app instead of the default app & that did allow me to send from all accounts, except for the TalkTalk account. Same settings, but a different app — weird! Just need to raise the issue with TalkTalk now, as the problem seems to be at their end, rather than with the phone or Orange. Not sure how successful that'll be!
Separate names with a comma.