When attempting to send an email from the server, you will see this failure in exim_mainlog:
2021-03-29 18:34:32 1lQs2K-0008Jy-7f sendgrid_login authenticator failed H=smtp.sendgrid.net [IP_ADDRESS] 535 Authentication failed: Basic authentication is not allowed with 2FA enabled. To fix, see https://sendgrid.com/docs/for-developers/sending-email/authentication/#basic-authentication
This is not a cPanel-related issue and is related to recent changes to the authentication requirements in SendGrid's API.
As of Q4 2020 two-Factor authentication is required and all Twilio SendGrid API endpoints will reject new API requests and SMTP configurations made with a username and password via Basic Authentication.
You can read more about this issue here:
Here you can find the workaround suggest by SendGrid official documentation:
Please note that, if you require further assistance with this issue you must reach out to SendGrid's support channel: