There was an error sending your message: Failed to add recipient: firstname.lastname@example.org [SMTP: Invalid response code received from server (code: 550, response: Verification failed for <email@example.com> No Such User Here Sender verify failed)]
I am getting this error for this domain only all other existing domains are working fine in horde webmail.
1 thing i have noticed which is : <firstname.lastname@example.org>
While i have logged in using email@example.com mail id then why this mail id <firstname.lastname@example.org> is showing in horde webmail
How are you logging into webmail? Are you going through cPanel and then clicking the webmail link, or are you opening webmail directly? When you do log into webmail, do you enter a full email address as the username or just your cPanel username? (Entering just the cPanel username to access webmail can cause behavior like this...)