Yahoo:
- Connection rate: 10/s.
- Message rate: 100/s per IP.
- Strict on DKIM alignment — d= must match From or strict subdomain.
- Yahoo Sender Hub (sender.yahooinc.com) for FBL registration.
- Bulk requirements mirror Gmail's 2024 requirements.
Microsoft 365 (*.protection.outlook.com):
- Very low concurrency: 3–5 connections per IP.
- Sensitive to bursty traffic — pace evenly.
- SNDS + JMRP enrollment is mandatory for visibility.
- Often returns vague
5.7.1 Service unavailable— usually means IP reputation. - Will throttle silently — watch for queue buildup on
*.protection.outlook.comeven when no bounces appear.
Shape table:
["yahoo.com"]
connection_limit = 10
max_message_rate = "100/s"
idle_timeout = "60s"
["*.protection.outlook.com"]
connection_limit = 4
max_message_rate = "40/s"
idle_timeout = "30s"
ProtonMail, Apple iCloud, regional providers: start conservative (2 conn / 10 msg/s) and ramp based on observed behavior.
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