I've been using Nisus Email for a couple of months now (in OS 9), but so far attachments seem to arrive at Windows users without a hitch. However, I can't say I've pushed things too much in that regard.
Hope you get on OK with the app. - it's taken me a while to get used to it, but compared to the other clients I've settled on (or tried to) over the years (Outlook Express, Netscape 4.7x and 7, Apple Mail and Sweetmail on the Mac; Outlook 2000 and Netscape 7 on the PC) I prefer the simple and very quick interface while putting up with the limitations...
...OK, I admit it - for HTML mail that NE can't clean-up and display properly, and mailing lists that need threading support I use Mozilla 1.02