Hello, I am having trouble sending emails from Thunderbird after changing my mail server's hosting provider and reconfiguring the existing email accounts. POP3 works fine, but SMTP does not. I keep getting the following error: An error occurred while sending mail. The mail server responded: "The use of IP (xx.xx.xx.xx) in HELO is forbidden!". Please check the message recipient "yyy@gmail.com" and try again. Same message appears if I try to send to other email addresses, not just Gmail. The IP address here is the actual IP address of my mail server. In my SMTP settings, I use the most basic settings: port 25, mail.domain.tld, No authentication + Password transmitted insecurely. I also tried various other options, including port 465, SSL/TLS, Normal password / Encrypted password. I am using the settings as suggested by the hosting provider. The mail server has DMARC and DKIM activated. My hosting provider's support says that the issue is with Thunderbird configured incorrectly using the IP address instead of mail server name, but I never entered the IP address anywhere. I believe Thunderbird resolves the mail server name to the said IP address and sends that in the HELO. Can you advise where this message comes from (my mail server or the mail server of the recipient) and what I can do to fix this? Thank you! Best regards, Stanislav