Queue depth alone is noisy. A high-volume sender can have many healthy queued messages for a short time.
Track at least:
- total queued messages;
- age of the oldest message;
- destination concentration;
- repeated SMTP status codes;
- retry counts;
- queue growth rate.
A sudden queue dominated by one destination points somewhere different from a queue growing across every domain.
Alert on conditions that require intervention, such as sustained growth or old messages, rather than every temporary retry.