Plesk stores most of its information in the psa database, so getting information out can usually be done via an SQL query. To export a list of all domains with emails enabled, run the SQL query below:
select distinct domains.name from mail,domains where mail.dom_id=domains.id order by name;
If you want to store the output in a text file, you can add INTO OUTFILE ‘/location’ into the query:
select distinct domains.name INTO OUTFILE '/tmp/output.txt' from mail,domains where mail.dom_id=domains.id order by name;
Lastly, if you wanted to put this into a Bash script (e.g; to send over a list of mail-enabled domains to a spam filtering server) you can run:
plesk db "select distinct domains.name INTO OUTFILE '/tmp/output.txt' from mail,domains where mail.dom_id=domains.id order by name;"
The above command can be run by anybody with root privileges and will output a list of all mail-enabled domains to /tmp/output.txt. Note that MySQL cannot overwrite existing files, so you will need to add a line into your script to remove the file before running the command above.