What is greylisting and how does it affect queues?

Greylisting is an anti-spam technique where recipient servers temporarily reject mail from unknown sender/recipient combinations. Legitimate servers retry; spam bots often don't.

How it works:

1. Unknown sender sends first message
2. Server returns 4xx (usually 451) telling sender to try later
3. Server records the sender IP, from address, and recipient
4. If the sender retries after a delay (often 5-15 minutes), they're "greylisted" and future mail proceeds normally

• Impact on queues:
• First-time messages to greylisting servers always get deferred. Your queue holds them until retry.
• Delivery to new recipients takes longer (the greylist delay plus your retry interval).
• Once past greylisting, subsequent messages to the same recipient flow normally.

For ESPs sending to many recipients, greylisting causes initial delays but doesn't significantly affect throughput once sender/recipient pairs are established.

From sender perspective: Greylisting is invisible if your MTA retries properly. Messages just take a bit longer for new recipients. Ensure your retry policy allows for typical greylist delays (at least 5-minute minimum backoff).

It's a trust test. Real ships return when told to wait; smugglers often don't.