Can I use cPanel's SRS (Sender Rewriting Scheme) feature to forward email as a SendGrid verified Sender Identity?
No, it is not possible to use cPanel's SRS feature to forward mail as a SendGrid verified Sender Identity.
cPanel's SRS feature will not work for this purpose because it only adds a Return-Path header to the message when forwarding the message with the explicit and sole purpose of allowing SPF verification to pass. It will not modify any other part of the message.
SendGrid does not make use of the Return-Path header to determine the "verified Sender Identity" so the SRS feature cannot serve the purpose of forwarding messages on behalf of a SendGrid "verified Sender Identity". The SendGrid documentation states that the "From" address is used for the purpose of checking the "verified Sender Identity". This indicates that they are checkin the "From" header which the SRS feature intentionally does not modify.
If you are seeing an error message in your Exim Logs similar to the following example, the SRS feature will not solve the problem. Instead you would need to contact a systems administrator that has the skills, training, and expertise required to create a custom Exim configuration that would enable you to rewrite the headers of the message in a way that is compatible with the SendGrid requirements. You may instead have the customization setup so that the message is actually re sent entirely rather than being forwarded. The exact details of how this issue would be handled would need to be determined by your systems administrator.
550 The from address does not match a verified Sender Identity. Mail cannot be sent until this error is resolved. Visit https://sendgrid.com/docs/for-developers/sending-email/sender-identity/ to see the Sender Identity requirements
More in-depth details about how cPanel's SRS feature works can be found in the following article: