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Deliverability

Why messages end up in the wrong folder, and what a receiving server has already decided before it reads a word.

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Do transactional emails need an unsubscribe link? The Gmail and Yahoo rules, applied to application mail

The 2024 Gmail and Yahoo sender requirements made one-click unsubscribe mandatory for bulk senders, and left developers unsure whether a password reset now needs an opt-out. It does not, but the reasoning matters, and the other requirements apply to every message you send.

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Hard bounce vs soft bounce: what the SMTP reply codes mean, and what to do with each

A bounce is a receiving server's answer, and the answer has a structure. The first digit says whether to retry, the enhanced status code says why, and the text is for humans. Here is how to read the common ones, which addresses to suppress, and which rejections are about you rather than the recipient.

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Why your password reset emails land in spam

A message can pass SPF and pass DKIM and still fail DMARC, because alignment asks a different question than either of them. Here is how to tell which check is failing and why.