You can select Atmail as webmail client in a dropdown menu, however whenever webmail.domain.tld is opened, the previously selected webmail client is displayed.
Atmail support have been discontinued in Plesk 11.5. Please refer to this article. The package '
psa-atmail' might still be present in the system, showing up as an option for webmail client selection.
Locate and remove '
psa-atmail' package, for RPM-based systems:
rpm -q psa-atmail rpm -ev psa-atmail
for DEB-based systems:
dpkg -l psa-atmail dpkg -r psa-atmail