If you are having the 554 problem with only one domain, try what I did, and it might fix it also
Hmmm... I haven't tried adding additional domains yet. I don't think it will matter, though, because I totally removed and reinstalled Postfix and ISPConfig trying to fix the problem, with the same result. But I'll try setting one or two of my other domains up tomorrow to see if the same thing happens.
Thanks for the tip!