There is no Thunderbird antispam system. Thunderbird does not use the word Spam or mark messages as such. Your email provider probably does. You should ask them.

Hi,

Thank you for your email.

I can send emails with attachments from my email if I use my browser. So I do not think that the issue comes from our email provider.

We have contacted them and they said that it has probably something to do with the program we use. (Thunderbird)

I have been using Mail Merge ADD-ON to send a 100 mails to my clients as a reminder but I cannot imagine that this has something to do with it. Since no spam had been send out nor we exceeded the volume.

Looking forward to your reply.

Cheers

When you send using your providers Webmail it does not use the SMTP server so that attempt at a comparison is meaningless. Ask your provider what it is about your messages that their server does not like.

As Airmail said, Thunderbird does not use the word "spam". The built-in junk controls use the word "junk" and they don't modify the message, but just add a flame icon.

If you're seeing any mention of the word "spam" it is coming from outside Thunderbird.

It may mean that your ISP's user IP address range has been blacklisted, or that some arbitrary rule or configuration has decided that some aspect of the messages as composed by Thunderbird (such as inclusion of a non-routeable IP address) makes your messages fail their test.