Already a few days, I noticed people not answering my emails. I started to investigate and found out that my emails were landing in the Gmail spam folder. It took me some time to figure out that TheBat was the problem: sending the same message from the webmail was the key to finding out that. Trying another email client definitely told that TheBat was the issue, as with that client it worked.

I read "Gmail requires emails to include both plain text and HTML versions in multipart/alternative format to avoid spam filtering."

How can I make sure of this ? To be honest, I changed a few things, also the charset to UTF-8 and added X-Mailer: The Bat! (v11.4.3) Professional, and suddenly all mails that were going to the Spam folder were going to the Inbox. Now I am not sure which change was the one that made the difference