The problem is with your SMTP settings, and truthfully, may not be fixable. The issue is that your ISP (the one you use to actually connect to the internet) is requiring you to use THEIR SMTP server, and requiring you to authenticate through them. But even if you get those settings correct, you may run into issues when you are connected to another network - either 3G or wifi.
Originally Posted by Shawn Bouchard
You can thank the spammers and spoofers for creating this mess... and the ISPs for locking everything down to try to prevent the spammers and spoofers from doing their deeds on the ISPs servers.
I have multiple email addresses configured on my iOS devices - one that always works is a gmail.com address. Another one is my personal web domain which is hosted by GoDaddy. GoDaddy is unique in that they offer "relays" that you can use to connect to their SMTP server through any ISP and authenticate and send email. I am not sure what your ISP offers in the way of relays or similar. Maybe their website or their tech support can help you get it sorted out... it's one of those things that makes you crazy... and each ISP has their own way of dealing with spam, spoof and authentication.