I strongly recommend to make a backup of the DB before you try this! You also need to delete the appropriate records from isp_dep and isp_nodes.
I could not find any reference to webmail.domain.com or domain.com in isp_nodes. I could not also tell which record(s) in isp_dep was linked to webmail.domain.com or domain.com because all the values in this table are numbers. What I did was to delete the record in isp_isp_web which had domain.com in it. This seems to have solved the problem because I can now create webmail.domain.com. ISPConfig seems not to have been adversly affected by the poking around in its database. Thanks a million for the help Falko.
