When the number of email notifications sent out from Service Desk is huge, the delivery of emails to recipients will become slow. Though the SMTP server processes the emails faster, there will be delay at pdm_mail_nxd in handing over the emails to SMTP server. The emails which are yet to get processed will be stacked up in the mail_queue folder.
The processing speed of emails by pdm_mail_nxd will be based on the number of concurrent connections to SMTP server. By default this was set to 3 using NX variable mail_max_threads.
Administration tab -> Options Manager -> Email -> mail_max_threads
This value can be increased up to 10 (or) 20 based on the number of concurrent connections the SMTP server can accept.
Recycle of Service Desk service is mandatory after changing this value in Service Desk options manager.