Wow. Lee Lukehart at SavvyData has built a cool utility to check the possible options for your mail server settings. You enter the settings you're using (username, server address, mail address, ports, etc.) into a script in his utility and it tests all sorts of combinations to try and tease out which settings work and which are causing errors.

His post about this also has some great tips for formatting addresses correctly.

Definitely worth checking out!

(And remember, in fmSpark, mail is sent using the logged-in user's mail settings under Settings / Users / Mail Account, NOT using the default settings. Hope that helps)