What tools help debug delivery failures?

Delivery debugging tools help diagnose problems:

SMTP testing:

telnet: Basic SMTP conversation testing. openssl s_client: TLS-enabled SMTP testing. swaks: Swiss Army Knife for SMTP. mxtoolbox: Online SMTP diagnostics.

DNS tools:

dig: Query MX, SPF, DKIM, DMARC records. nslookup: Basic DNS queries. mxtoolbox: Online DNS analysis.

ESP and platform tools:

ESP bounce logs and reports. Delivery status dashboards. Message search and trace.

Postmaster tools:

Google Postmaster Tools: Gmail reputation and delivery. Microsoft SNDS: Outlook.com data. Yahoo Postmaster: Yahoo/AOL insights.

Header analysis:

Google Admin Toolbox. MXToolbox header analyzer. Manual header inspection.

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