Yandex

4.4.2

Yandex timed out while receiving the message. Retry with backoff. If it repeats, reduce concurrency to that host.

4.2.1

This Yandex recipient hit a message rate limit. Wait and retry. Slow sends to that mailbox if it repeats.

4.7.1

Yandex says the service is temporarily unavailable. Retry with backoff.

5.7.1

Yandex blocked the recipient account for policy violations. Suppress the address.

5.7.1-2

Yandex says this user does not exist. Treat it as a hard bounce and remove the address.

5.3.0

Yandex returned an unknown 551 error. Retry once. If it repeats, verify the address and contact Yandex support with the full bounce.

5.2.2

This Yandex mailbox is full. Retry later, or ask the recipient to free space.

5.7.1

Yandex rejected this under spam suspicion. If you rarely send there, check signup forms for bot abuse. Otherwise clean content and list quality.