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?
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: http://tools.ietf.org/html/rfc822) and shouldn't ever hurt your deliverability.
Also, related question: Custom header mail() and spam score
来源：https://stackoverflow.com/questions/10684097/can-custom-email-headers-trigger-spam-filters