This is what I did: Create a site www.domain.tld with DNS and DNS-MX checked. In this site created a user webXX_user1 with e-mail address firstname.lastname@example.org and Administrator and catchAll-Email checked. Create a site user2.domain.tld with DNS checked. In this site created a user webYY_user2 with e-mail address email@example.com and Administrator and catchAll-Email checked. The problem is that e-mails addressed to user2 end up in the mailbox of user1. After I first discovered that, I also checked DNS-MX of site user2.domain.tld but that doesn't make a difference. Two observations: When creating a sub domain in this manner, ISPConfig constructs a default e-mail address in the primary domain for users of the sub domain and it can not be changed. Both the primary domain and the sub domain can have the catchAll-Email checked, while e-mail addresses for both domains "live" in the primary domain. I can imagine conflicts arising from that. Any idea what's going wrong here and how I can solve it?