Unable to send email from a mailbox in Plesk the following error is shown:
Sometimes, messages can be sent, but they are bounced back with the content below (the same error can be found in
Diagnostic-Code: smtp; 550 The from address does not match a verified Sender
Identity. Mail cannot be sent until this error is resolved. Visit
see the Sender Identity requirements
The Server Identity has not been verified by SendGrid and due to this, the SendGrid servers are not accepting the emails that reach them and are sending them back instead.
Contact SendGrid support in order to receive information related to the ways in which you can verify the identity of your Plesk server in the eyes of SendGrid server.
One of the ways in which such an issue can be resolved is described in the following official SendGrid guide: