Whilst i write this i'm on my phone walking to work. Before I tried sending an email from my clear email address in outlook 2010. It seemed to receive emails fine, but I was getting error 503, wanting me to authenticate myself. Never before have I had the outgoing server setting enabled to do that, and was without issue until today. Perhaps it could be an email server issue, but was wondering if anyone else was having this issue.
It probably is the SMTP setting, but without telling us what SMP server you're using it's impossible to know.
Virtually every ISP either blocks access to their SMTP servers from outside their network and/or requires SMTP AUTH to be used which is why you will normally find it easier to use the SMTP server for the ISP you're using when you want to send the email - If you were using Vodafone for example for mobile you could also use their SMTP server to send email.