Check manual page of check_mailboxes

IMAP Mailboxes
Distribution Official part of Checkmk
License GPLv2
Supported Agents Active
This active check logs into an IMAP or EWS account and optionally monitors the age of the oldest or newest mail as well as the total count of mails in one or multiple mailboxes.

There is a caveat though: Many mailservers don't support retrieving mails sorted by age and it would be very costy (in terms of traffic and check time) to retrive all mails. To work around this, the check retrieves a configurable number of the least recently and most recently modified mails from the server and checks the age of those. Unless mails get moved between mailboxes, the least recenly modified mail should also be the oldest one and the most recently modified mail should be the newest.

As for EWS the mailbox to be contacted can be specified by an email address which in contrast to IMAP might be different from the username provided with legacy OAuth credentials (but doesn't have to be). For that case as well as for (non-legacy) OAuth2 credentials using ClientID and TenantID this email address (a.k.a. 'PrimarySmtpAddress') can/must be specified in the setup for this check (available for EWS only).