Things to check if not receiving emails from Jira:
Check your Mimecast Digest.https://community.mimecast.com/s/article/The-Digest-Email-373703617
Check your Junk mail folder
Check for any email rules in your inbox
Do you get an email if the issue is shared with you manually?
Open the issue view of the request.
Click the share icon ().
Enter who you want to share the issue with and a message to send them.
Click Send.
Are you getting any other notifications from Jira?
Example - from a different project or a different event like issue commented or @mentions.
If so, please submit a support request with the ticket number that for which you are NOT receiving notifications.
Is your Atlassian account set up to receive notifications?
Make sure the appropriate boxes are checked.
4. Are you a watcher on the tickets that you are not receiving notifications for @mentions?
There is a chance that if a user is already involved on a ticket the @ mentioning will not come through. Jira won't send 2 notifications for 1 ticket to the same user. If they are not watchers or participants on the ticket below is a blog on using watchers & @ mentions effectively. There might be some insight on what is happening.
Using watchers and @mentions effectively in Jira
5. After checking all of the above and you are receiving NO notifications from Jira then you then we will need to have an Exchange admin verify if there is any mail coming to this account from jira@fergusonprod.atlassian.net so we can confirm we are not blocking the emails.
6. If we are NOT blocking the emails and the Exchange admin does not see any traffic to these accounts from jira@fergusonprod.atlassian.net you will need to engage Atlassian support.