If your mail server was working fine for some time, but suddenly you find the following error in the mail log,
Aug 25 20:25:24 mx postfix/trivial-rewrite[3313]: warning: virtual_alias_domains: proxy:mysql:/etc/postfix/sql/mysql_virtual_alias_maps.cf: table lookup problem Aug 25 20:25:24 mx postfix/trivial-rewrite[3313]: warning: virtual_alias_domains lookup failure Aug 25 20:25:24 mx postfix/submission/smtpd[3464]: NOQUEUE: reject: 451 4.3.0 <user@your-domain.com>: Temporary lookup failure; proto=ESMTP Aug 25 20:25:24 mx postfix/submission/smtpd[3464]: Temporary lookup failure
It’s likely that your MariaDB/MySQL database stopped somehow. You can use the following command to check when your database server stopped.
sudo journalctl -eu mariadb
or
sudo journalctl -eu mysql
A common cause for this situation is that your server is out-of-memory. Check if your server has enough memory.
htop
or
free -m
This error can also be caused by too many connections to the MariaDB/MySQL databases, so it can’t handle more queries. To fix this, you can increase the number of connections MariaDB/MySQL can handle.
sudo nano /etc/mysql/mariadb.conf.d/50-server.cnf
Find the following line in the Fine Tuning section.
#max_connections = 100
Uncomment it and change the value to 500.
max_connections = 500
If your server has lots of RAM, you can change it to a bigger number like 1000.
max_connections = 1000
Save and close the file. Then restart MariaDB/MySQL.
sudo systemctl restart mysql
