Can custom email headers trigger spam filters?

Question

We do a lot of mailing and we use Sendgrid to give it unique arguments and categories. But for storage, archiving, filtering and search capabilities, I would like to give them some other custom headers too, something like a json format of this array

array('user' => 300, 'item' => 8842, 'country' => 'Croatia');

Now, is it possible for such custom headers to hurt the passthrough rate of our emails? In other words, is there a chance of our emails triggering spam filters due to having such custom headers?

Gony

Answer 1

The convention for user defined headers is to prefix them with an X-. For example

X-my-custom-header: foo

This has been a part of the email spec for a long time (see: https://www.rfc-editor.org/rfc/rfc822) and shouldn't ever hurt your deliverability.

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Also, related question: Custom header mail() and spam score

Jameljamerson

To be precise, the use of X- for custom headers isn't/wasn't specified in RFC 822. All it said was that future extensions wouldn't add names that started with X-. 822 was obsoleted in 2001, and the subsequent standards (2822/5322) dropped this text entirely. You've always been free to define any names you want, as long as they're syntactically correct. – Equipollent