If multiple number of notification emails are sent out at the same time, some of them will be stored under NX_ROOT\Service Desk Manager\site\mail_queue folder and wait for the next trigger to connect to a mail server.
CA Service Desk Manager : 17.3.X
There is no setting of mail server connection interval, i.e. we are not able to control how frequently Service Desk Manager connects to a mail server.
When many emails are sent out and a value of the option mail_max_threads is reached, all threads status will be set to "Busy". Once the status is changed from Busy to "Ready", the emails stored under the mail_queue folder will start to be sent out.