I use gmail to send new posts to my moblog, and I also have a POP3 account set up exclusively for my phone - however, I've noticed that recently my POP3 server hasn't been authenticating connections properly, and therefore has dropped them - it's happened when sending emails from my phone (both via GPRS/3G and wifi) and more recently, it happened on my desktop client, so it looks like it was a server-side issue and not a more general problem... But I do think that at least at one point, Pocket Outlook was being a bit stupid and not authenticating correctly before trying to send emails, because I had a LONG telephone conversation with a TM tech about the problem the night it was happening, she gave me the T-Mobile SMTP details and I tried those out - and got the same problem. It resolve itself the day after, but I could never figure out why it was doing it, other than a guess that Pocket Outlook might have been at least partially at fault.
If your POP3 client can't authenticate correctly before sending outbound mail via SMTP, you'll get this rather cryptic error message back (at least, in Pocket Outlook, and I got it in my desktop Outlook client when I had the problem with my main email account):
Error 550: Invalid Recipient
Only by doing some packet sniffing did I actually realise what the root problem was, and I emailed my webhost and informed them of the problem, they 'tweaked' some settings and da-daaa: it all works perfectly again. I still use gmail though, even when my phone was having issues with my own POP3 account, gmail worked fine every time.