[IronPen]

Can it be done? Thunderbird allows it, but I like Mail.app quite a bit and would like to stick with it. The problem is that some mail clients (Outlook, in particular) that use HTML formatting cause other mail clients to display the message font in really tiny text.

[cpac]

there's not a preference or anything obvious, but my suggestion is to just keep the "make font bigger" toolbar button visible in your tool bar for those messages with tiny text.

I do, and use it when I get those tiny outlook emails - not too much of a hassle to click the button once or twice....