Mailbox Providers

• Apple iCloud Mail — Postmaster Requirements

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

• B2B / Corporate-Gateway Deliverability

  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.

• Comcast / Xfinity — Postmaster: Error Codes, Rate Limits, Unblocking

  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.

• Gmail Email Sender Guidelines

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

• Gmail Filtering Internals

  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.

• Gmail SMTP Errors and Troubleshooting

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

• GMX / WEB.DE (United Internet) — Postmaster Requirements

  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.

• Microsoft Escalation Channels

  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.

• Microsoft Filtering Internals (EOP / Defender for Office 365)

  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.

• Microsoft Outlook.com Sender Requirements

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

• Provider Escalation and Mitigation Channels

  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.

• Yahoo Complaint Feedback Loop (CFL)

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

• Yahoo Email Deliverability Performance Feeds

  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.

• Yahoo Sender Requirements & Best Practices

  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.

• Yahoo SMTP Error Codes

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