How do Yahoo “deferred due to policy” messages appear?

Yahoo "deferred" messages have characteristic patterns:

Typical format:

"421 4.7.0 [TS01] Messages from [IP] temporarily deferred due to policy." "421 4.7.1 [TS02] Messages deferred due to excessive unknown user rate." "421 4.7.0 [TS03] Messages temporarily deferred."

TS code meanings:

TS01: General throttling, often reputation-based. TS02: Too many invalid recipients. TS03: General temporary deferral.

Characteristics:

4xx codes (temporary, retry). Less detail than Gmail. Policy reference without specifics. IP address noted.

Response:

Reduce sending rate. Wait and retry. Review list quality (for TS02). Check Yahoo postmaster for reputation data.

Yahoo's deferrals are polite but vague. Slow down and investigate your sending practices.