I prefer a PIM-type client, so i'd use Kontact in KDE or Evolution in Gnome. My vote would be for Kontact, since that would give the users choice to use either standalone Kmail or Kontact if they want the whole PIM suite.
I voted for Thunderbird. But, as they are for two different 'markets', I'd like to see Evolution supported also. Thunderbird is akin to Outlook Express, and Evolution is akin to Outlook. I have both installed on my Windows XP partition. But, if you really only want to have one e-mail client, then I would have to say Thunderbird.
frankly, Thunderbird is unsuitable and insufficient for business.
a full PIM suite is absolutely essential, and thunderbird simply isn't that.
your two only choices are evolution and Kontact.
when i ran an engineering development company i used suse on my laptop. i started with thunderbird because i was used to it under a previous windows laptop, but i quickly found that kontact was a life-saver.
given that sabayon is KDE centric i would opt for kontact rather than evolution.