How do I configure sending limits and queues?

Sending limits protect your reputation and prevent overwhelming recipient servers. Queue configuration determines how long messages wait and when retries occur.

Connection limits: Control how many simultaneous connections your MTA opens to any single destination. In Postfix, use smtp_destination_concurrency_limit (default 20). Too many connections can trigger rate limiting at the recipient's end.

Message rate limits: Limit messages per connection or per time period. Postfix uses smtp_destination_rate_delay to add delays between messages to the same destination.

Queue lifetime: Set maximal_queue_lifetime (Postfix) to define how long undeliverable messages stay in queue before bouncing. Five days is standard, matching RFC recommendations.

Retry intervals: Configure minimal_backoff_time and maximal_backoff_time to control retry spacing. Start with short intervals (5 minutes) and extend to longer waits (4 hours) for persistent failures. This is called exponential backoff.

Example Postfix settings:

• smtp_destination_concurrency_limit = 10
• maximal_queue_lifetime = 5d
• minimal_backoff_time = 300s
• maximal_backoff_time = 4000s