The problem: Sometimes I do not check irssi for messages over days or weeks. If I receive a private message or a mention on some channel, I will not read them.
To solve this I found the script email_msgs.pl. It was made for sending a mail from irssi host when you receive some kind of messages. Very simple and useful.
Below I show how to use it to send mail when you receive private messages or a mention.
Step 1 - install and enable the script
# apt install irssi-scripts libemail-sender-perl
$ ln -s /usr/share/irssi/scripts/email_msgs.pl ~/.irssi/scripts/autorun
Note: The last command creates a link of email_msgs.pl inside the directory ~/.irssi/scripts/autorun.
Step 2 - configure
/set email_msgs_to_address firstname.lastname@example.org
/set email_msgs_from_address email@example.com
/set email_msgs_subject [irssi] message
/set email_msgs_away_only off
/set email_msgs_detailed on
/set email_msgs_interval 300
/set email_msgs_pri_r_msgs on
/set email_msgs_pri_s_msgs on
/set email_msgs_pub_r_msgs off
/set email_msgs_pub_s_msgs off
/set email_msgs_mentions on