Problem with outgoing mailserver (smtp)

Yesterday morning, we got an alarm because the smtp service did not work properly. The cause was found to be that from various PCs from, among other countries, China, connections were built up with one of the smtp servers, without actually sending an e-mail, as a result of which all these connections were left hanging. If the number of standing connections became too high, the server became unusable for others. This service has to be be accessible from the Internet, because of authenticated SMTP. After a few PCs had been blocked, at 15:39 Fail2ban was turned on, that is also used to protect other services accessible from the Internet, such as mailservers, Linux loginservers, WWW-servers and GitLab servers.