I've this problem too. The solution is to ensure the domain has the "Emaildomain" option set to YES.
Emaildomain already was enabled, so this was not the problem.
For MySQL, I am not sure. I guess you need to ensure the number of databases is set to >0
This is what did the trick for me - setting accountlimits to zero didn't give unlimited accounts to me, it disabled them for this account. Entering 300 did it :-D
I must say I'm VERY happy now, thank you very very much!
Cheers
tcs
P.S.: Please could someone be so kind and add a [solved] tag to my first posting? I can't edit it anymore. thx :-)