When clicking on Domains > example.com > Mail Settings, the following warning is shown:
PLESK_WARN: Warning: Permission Denied
Mail tab is missing in the subscription menu.
Mail management functions are disabled in Tools & Settings > Mail Server Settings.
On Linux the following error can be found in
[2018-03-16 16:53:33] ERR [panel] PleskPermissionDeniedException: Permission Denied.
Mail management functions are not available bеcause they are disabled in Plesk mail settings.
Log into Plesk as an administrator and go to Tools & Settings > Mail Server Settings.
Note: if the access to admin account is not available, contact server administrator.
Select the checkbox Enable mail management functions in Plesk.
Click OK to apply the changes.
Note: In case Mail service for the license is disabled in Partners Central, activate it in order to prevent the issue from happening again. If the license was bought not directly from Plesk, please consult the reseller.