What causes emails to get stuck in queue?

Messages stuck in queue indicate delivery obstacles. Common causes:

Recipient server issues: The destination server is down, overloaded, or returning temporary errors. Your MTA keeps retrying until the server recovers or the queue lifetime expires.

Rate limiting: The recipient is accepting messages but throttling your connection. Each attempt delivers a few messages before hitting the limit, and the rest wait in queue.

DNS problems: Can't resolve the recipient's MX records. Without a destination address, delivery is impossible. Check your DNS resolver and the recipient domain's records.

Authentication failures: If your MTA requires authentication to a smart host and credentials fail, all outbound mail queues.

Network connectivity: Firewall changes, routing issues, or port blocks can prevent connections to destination servers.

Content-based deferrals: Some providers defer suspicious content rather than rejecting outright. The messages queue for retry but may keep hitting the same wall.

Investigate stuck queues by checking the deferred or bounce logs. Error messages explain why delivery failed. Addresses stuck for days may indicate permanent problems requiring manual intervention.