For all outbound email notifications, Service Desk Manager looks at the values specified for the option "NX_MAIL_SMTP_HOSTS" in the options manager.
This options allows you to provide a space separated list of server names (or IP addresses) for the SMTP servers to which you can connect.
ServiceDesk will try connecting to the first server, if it fails tries second and so on.
After reaching the end of list if the specified mail servers are not reachable, the email gets queued in the mail_queue folder.
If you wish to attempt to force the pdm_mail process to process emails from the queue you can run the command:
pdm_mail -c PROCESS_QUEUE
If the mail daemons have given up attempting to reconnect to the mail server you may want to manually terminate the pdm_mail_nxd.exe and pdm_mail.exe processes. After termination they will automatically restart and attempt to establish a connection to the mail server. Items in the mail queue will attempt to process.