Some clients have reported an issue with receiving email notifications from xMatters where their email was either delayed or sent to their Junk folders.
This is often due to the fact that when a major incident occurs, xMatters can send out a lot of notifications in a very short amount of time. Unfortunately, this can occassionally mimic the behavior of a spam email server - especially when xMatters is sending an email to a lot of people in the same company. Many corporate email servers contains tools such as Norton that look for this sort of behavior and tag the emails as spam, which can either delay the emails, prevent them from being delivered, or send them to a Junk folder.
To prevent this from happening in your organization, and to ensure that your xMatters emails are delivered promptly, talk to your email server admin or IT department and make sure that they whitelist the email server and domain from which xMatters is sending your emails.