“Activate mail service on domain” button is greyed out

Connect to the server via SSH/via RDP.

Create a backup of Plesk database:

Plesk for Linux

# plesk db dump psa > psa.dump

Plesk for Windows

C:> plesk db dump psa > C:psa.dump

Login to Plesk database:

Plesk for Linux

# plesk db

Plesk for Windows

C:> plesk db

Find ID with disabled mail service (in the example below, it is 167 ):

mysql> select * from DomainServices where type='mail' and dom_id=(select id from domains where name='example.com');
+-----+--------+------+--------+---------------+----------------+
| id | dom_id | type | status | parameters_id | ipCollectionId |
+-----+--------+------+--------+---------------+----------------+
| 167 | 61     | mail | 2      | 229           | 167          |
+-----+--------+------+--------+---------------+----------------+
1 row in set (0.00 sec)

Where example.com is a main subscription's domain.

Change mail service status using id from previous step:

mysql> update DomainServices set status=0 where type='mail' and id=167;
Query OK, 1 row affected (0.00 sec)