In order to determine which emails are accepted by recipient servers, mailbox providers follow RFC protocol on email formatting and content. In the last number of years, in order to crack down on spam, providers have become stricter in their email screening. On one hand, it does help with spam control but, on the other, legitimate senders are being penalised for slight errors in their email formatting. In order to compensate for this, instead of just rejecting an email that is poorly formatted, mailbox providers have tried to automatically fix the formatting issues. By fixing any of these issues, the mailbox provider is violating the DKIM signature, which can lead to further receipt issues as the email is no longer properly authenticated.

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