You should consider using Windows Server's IIS SMTP server as a local mail relay on your MOVEit Central system any of the following conditions apply.
MOVEit Central can send files by email, or send email alerts on completion of a task. These notifications happen as part of the task run, so MOVEit Central must wait for each message to be sent before the task is completed. When MOVEit Central is dealing with busy email servers or large numbers of recipients, tasks may run significantly longer than normal. To keep MOVEit Central from having to wait for each message to be sent, we can instead spool these messages to a local SMTP server which will queue up the messages and send them out to "real" email servers when they are better able to accept the traffic.