GLPI generates a serious amount of email notifications depending on your notification model. Heavily used instances can have tens of thousands notifications emails sent monthly, which is good to keep in mind when designing the infrastructure for a GLPI server.
GLPI keeps emails sent via its Mail queue. To view those emails proceed to Mail queue (Administration – Mail queue) and click on the trash icon. Once you see the list of sent emails, click on the wrench icon and adjust your view, so the list makes sense, it has a little poor view by default.
By default, emails are kept for 30 days, for most cases keep it as is. Consider lowering the limit if you have a big instance and relevant database table could get too big.
Automatic action taking care of cleaning the queue is „queuedmailclean“. The option to modify limit is evident – Days to keep sent emails.