One of the features I would love to see is the possibility to easily change the sender address. I have quite a few email adresses for different purposes (private, a few company/business addresses, university) that I forward to one unifying account. With YAM it's easy to maintain a multi address workflow - checking just one account, and setting the sender address according to the actual purpose. I would be glad if Iris can offer a similar workflow, too.
I think in Thunderbird it's doable rather okayish, too (dunno exactly as I don't use Thunderbird actively mayself), but nothing (known to me) in this regard is as easy as YAM (up to V 2.6 at least - IIRC it was changed in later versions and is one of the reasons I stick with V2.6).