email notification for Mentions stop arriving.

[Jim Boger]

We still recieve  the "You may have missed..." notifications but missing all the 'mention' emails that used to come individually .    sender is the same, "do-not-reply@trello.com' .

[Michael Pryor]

You can change how often you get notification emails on your Account settings page. You can also set your notification email preferences by clicking the notifications button in the header and selecting "Change notification email frequency". Options are "Never", "Periodically", and "Instantly." "Periodically" is the default, and any unread notifications in Trello are emailed out as soon as possible.

From: https://help.trello.com/article/793-receiving-trello-notifications

My guess is you set it to Periodically instead of Instantly.