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If you’ve followed our tips to prevent your email going into spam folders but you’re still having issues, your email formatting may be to blame.

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.

These are some factors to take into consideration when planning your email formatting:

Line Length Limits

According to the RFC Internet Message Format memo (RFC5322 Section 2.1.1), a line of characters in your email MUST not exceed 998 characters, but  SHOULD not be more than 78 characters (excluding the CRLF / line break). If your emails do contain more than these limitations, you will receive bounces and error codes such as:

If you are receiving any of the following error codes, it is likely that a line of your email has not been finished with a CRLF or Carriage Return Line Feed:

Header Fields

Although email headers can seem rather overwhelming and confusing, the only compulsory lines in the headers are the Date and Message-ID. Emails sent without these fields will commonly generate the following error codes:

Even if the email does get accepted by the recipient provider, anti-spam systems (such as SpamAssassin) may reject the email based on the same criteria.

Email Address Size Limits

It is also worth noting that there is a limit of 64 characters  in the local part of an email address (the part before the @ symbol), and a maximum of 255 characters in the domain part (RFC2821).  There is also a restriction on the length of an address in MAIL and RCPT commands of 254 characters. Since addresses that do not fit in those fields are not normally useful, the upper limit on address lengths should normally be considered to be 254. If you attempt to send to an email address (or more commonly, a reply-to email address with a Message-ID in the local portion) whose length exceeds this, you may encounter the following errors:

Non-Specific RFC-Based Bounces

If you receive an error code that doesn’t specify the reason why your email bounced, please don’t hesitate to get in touch with us for further information.

Some examples:

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