Thx for your reply, actually the hostname was only set to webserver1 and email. Not the FQN. I changed the /etc/hostname so it became now the same as /etc/mailname
and rebooted, but no change so far.
In ISPConfig under Sites and Email the corresponding server is selected with the FQN webserver1.domain.com and email.domain.com
I also can ping between the two servers using their FQN.
I'm still unsure why webserver1 thinks that it is responsible for maildelivery via maildrop instead of contacting email for mail delivery as it does with any other domain.
At some point in the past I had to rename the main serverdomain which included several manual changes in config files but I would be surprised if that's the issue since the problem domains are virtual (client) domains.