Recreating a subscription stall while creating the mail accounts.
Enabling the mail service after creating the subscription fails with the following error:
mailmng-outgoing failed: ERROR:__main__:column name is not unique
The outgoing mail control database already contains records about the subscription with the same name.
Connect to the server via SSH
Reconfigure mail subsystem:
# plesk repair mail
Create the subscription or enable the mail service.
In case the issue persists, try manually deleting the record associated with the subscription with the issue:
# /usr/local/psa/admin/bin/mailmng-outgoing --remove-subscription --main-domain-name=example.com