No idea what it was exactly.. but it doesn't happen anymore. ximian's website had an answer for this problem which dind't apply at all to me. I think I just used a different SMTP server (not mine, but my ISP's), although I could swear it happened with my ISP's server too. oh well, fixed now!
Since a few days, I cannot send mail using evolution 2.0.. I have made no changes to my configuration - did anybody else run into this trouble? It's not a problem with my ISP's email servers, as I tried a couple different ones (including my own smtp server) and got the same error, and I tired with mozilla thunderbird which worked fine.
RCPT TO <firstname.lastname@example.org> failed: Requested action not taken: mailbox unavailable.