Mailbox Providers

Google's requirements for delivering to personal Gmail accounts — authentication, spam-rate thresholds, one-click unsubscribe, bulk-sender rules, and the enforcement timeline.

Complete catalog of Gmail SMTP reply codes with their enhanced status codes, meanings, and fixes.

Yahoo Mail's requirements for all senders and bulk senders (authentication, one-click unsubscribe, 0.3% complaint threshold) plus recommendations on list hygiene, infrastructure, and reputation.

How Yahoo's DKIM-domain-based Complaint Feedback Loop works: enrollment via Sender Hub, ARF report format, and operational rules.

Yahoo Mail's SMTP error/deferral categories (4XX temporary, 5XX permanent, TS* deferrals, PH* content blocks) with causes and recommended actions.

Yahoo's server-side data feeds for senders: the Placement Feed (inbox/spam/folder/error/complaints) and Campaign Performance Feed (engagement metrics), with schemas and SLAs.

Outlook.com postmaster policies, the May 2025 high-volume (>5,000/day) SPF+DKIM+DMARC mandate (Junk folder or 550 5.7.515 rejection, depending on the Microsoft source), connection limits, and SMTP error code reference.

Reading X-Forefront-Antispam-Report and X-Microsoft-Antispam headers, SCL and BCL scales with per-level actions, compauth reason codes, how EOP/MDO layers filter, and Tenant Allow/Block List mechanics from the sender's perspective.

How Gmail actually decides placement — per-domain authenticated-sender reputation, ML/LLM spam and phishing classification, tabbed-inbox categorization, and the per-user personalization signals that make one-size diagnosis impossible.

Apple's published requirements for sending bulk email to iCloud Mail (mac.com, me.com, icloud.com), its filtering model, and its escalation path.

Sender requirements for GMX, WEB.DE and mail.com (United Internet): mandatory aligned DKIM, strict header/DNS rules, RFC 8058 unsubscribe, error-message format, and the CSA path.

Comcast (comcast.net / Xfinity) sender policy: full BL/RL/ES/DM error-code table, SenderScore-based rate limits, connection and recipient limits, blocklist removal, and FBL.

Delivering to corporate mail protected by secure email gateways — how Proofpoint, Mimecast, and Barracuda filter differently from consumer mailbox providers, their delisting paths, link-rewriting systems, and how to warm, diagnose, and measure B2B sends.

The cross-provider map of sender-support, mitigation, and unblock channels — Google, Yahoo, Apple, Microsoft — with the exact forms, required fields, eligibility gates, and the evidence to gather before any escalation.

The remediation map for Microsoft blocks: Office 365 anti-spam IP delist portal (sender.office.com), delist@microsoft.com for 5.7.511, the Outlook.com sender support form, SNDS/JMRP prerequisites, what each channel needs, and realistic timelines.