Generally when this occurs it is because the domain used to send the mails does not have the correct settings in DNS for DMARC and SPF. Those things are in no way related to Thunderbird, but I have seen a lot of folk that have hosted domains that do not have the email validations required by most major providers configured in DNS. I suggest you try putting your email domain name into this web site for some basic checking of it's setup. https://intodns.com/ You might also want to check it here https://check.spamhaus.org/ as this is a major spam blacklisting site. It might give clues if your domain is on a shared host with spammers and therefore your domains email reputation is rubbish, or it could be your sending of bulk mails has seen your domain declared a spammer. The USA in particular has some very lax rules about unsolicited email (SPAM)which when companies in the USA start emailing outside the USA can get them in trouble rather quickly. For instance in my jurisdiction unless a company can prove they have permission to send commercial email to an address (like I emailed you requesting something or subscribed to say a special offers newsletter) the corporate fines are huge (put you out of business huge). See https://www.acma.gov.au/avoid-sending-spam One thing I am not going to say is that is is the gmail host in the middle that is the deifinate cause of your troubles. If you are using the domains SMTP server instead of the gmail server then it is actually most unlikely to be the problem. Do not get me wrong, it might be the way you have set up Thunderbird. But from what you are said it does not sound like it. Hence the other suggestions in the first instance.