Can I have some help and/or suggestions with 1 strange mail forwarding issue?

Can I have some help with 1 strange issue?

I have email server and I have used Sendgrid - Postfix integration

My [email protected] email sends correctly to my Gmail account, from the snappymail webmail and Thunderbird. So the sending must be working. Also, I get 10/10 on the mail tester.

However, if I forward an email to [email protected] it doesn’t forward to it whenever I send an email to [email protected] from any random email. But, If I make it forward to another email on the server like [email protected], it forwards to it.

So, I am not sure what the problem is and why it doesn’t forward emails to my Gmail. Do you think the 600 permission I give to the file with the pass is causing the problem?