First, some background information…
CloudBees uses a 3rd party mail gateway (SendGrid) to improve the deliverability of messages to your mailbox. However, if there are delivery failures to your mailbox, the gateway will temporarily / permanently suppress further emails to your email address.
Notably if your server responds with a permanent delivery failure (SMTP error code 500+), then all further delivery to your email address is suppressed.
We sometimes see this if your domain’s primary mail-exchanger fails, and delivery switches to a secondary which isn’t configured the same way.
Ideally the secondary would respond with a temporary failure rather than a permanent one - however if it does respond with a 500+ error, then you are in the unfortunate position that all delivery is suspended indefinitely.
To have email delivery reactivated, please raise a support request with CloudBees support.