Unsure what to enter in “Sending Domain” field of WP Mail SMTP setup

Replies: 1

Hello, I’m trying to set up WP Mail SMTP for the first time. The test message is not working. The error says:

Versions:
WordPress: 5.8.3
WordPress MS: No
PHP: 7.4.27
WP Mail SMTP: 3.2.1

Params:
Mailer: sendgrid
Constants: No
Api Key: Yes

Debug:
Email Source: WP Mail SMTP
Mailer: SendGrid
The provided authorization grant is invalid, expired, or revoked

I suspect that I have not entered correctly in the “Sending Domain” field of WP Mail SMTP setup. Can you help me understand what is supposed to go in that field? Thank you.