For the sake of those who are seeking out solutions to this problem, I am adding the solution that finally worked for me. I have an email account through godaddy and I ran into this problem recently. There seemed to be no ryhme or reason for when it would spit out the dreaded "An error occurred while delivering this message via the SMTP server (null)" message. What made it worse is that every thread I found out there had no solutions to offer. Just like the uber-generic solution found here.
Nothing helped until I reconfigured my account with SSL settings. For some reason once I did this the problem went away and emails I couldn't send before, suddenly went out without a hitch. You may try changing the settings in your current email setup first, but you may have to start from scratch. I was so frustrated that I started out by wiping out my account entirely, before I tried the SSL change.
You can find the SSL settings, ports, etc... through your email server's website. On GoDaddy just log into your email account and it tells you what the settings are for pop and imap as well as what ports to use for SSL and non-SSL.