I have a SMTP server device that is an SMTP to SMS/Pager gateway that I need to send emails to. I have a successfully operational system using Thunderbird V24.2.0 on XP. But in expanding the system, new computers with Win Server 2012r2 were installed with Thunderbird V 52.6.0. The new Thunderbird was setup the same and it responds with "Outgoing server (SMTP) error The server responded: Invalid domain name". The SMTP connection is setup with the server name as an IP address and there is no DNS server so the client only has an IP address, and there is no SMTP user name, authentication or security. Investigating with wireshark, Thunderbird is connecting and sending "HELO [192.168.11.10]" (EHLO is not supported) and this is being rejected. I have done a Telnet to the SMTP server from the same machine and "HELO 192.168.11.10" works, but with the [ ] it is being rejected on the domain name error.

Is there a way to get Thunderbird to not use the square brackets in the HELO line? Alternatively, what version would I have to go back to get this functionality. Thanks.