I'm going to have to go for SimpleMail for this topic. I too was a YAM devotee, but as others have already mentioned, I think SimpleMail is starting to surpass YAM.
The new beta .23 version finally gives SimpleMail appicon support as well as also seeing a native MorphOS port. The Spam filter works great, the builtin HTML viewer works great (far better in my opinion than YAM's clumsy rexx scripts that feed the message into a webbrowser). Additionally SimpleMail is also far more stable and hasn't yet crashed on my setup like YAM was prone to do.
YAM still has a few advantages though having a better ARexx interface, but I'm sure SimpleMail is bound to catch up shortly. My only other gripe with SimpleMail at this moment is its' lack of CManager support...
Either way though, try out SimpleMail and YAM. They're both great mailers!