What did you check with your provider? Do they have a record of receiving them? If the messages are not getting to your providers SMTP server then maybe there is a problem with Thunderbird but you usually get an error message returned. If your providers SMTP server accepted the message but they still did not get delivered then there is nothing Thunderbird can do about that.

Is this messages addressed to single recipients? Is there any consistent pattern as to who do and who don't get your messages? Have your correspondents checked their spam folders?

If the messages are addressed to multiple recipients, do all, some or none of your correspondents get the messages?

Are you using separate discrete addresses, or a Mailing List? If you are sending to a large number of addressees, your smtp server may be refusing to handle greater than some pre-set number of addressees.

I sent 2 test emails to 2 people who were not receiving my emails before. I sent one through Thunderbird and one from Iprimus webmail.Sofar one person received the test email sent through Iprimus but not the one sent through Thunderbird.

Comparing web mail and SMTP server only confirms that you know your username and password. Other than that it is all different equipment on your providers end. Again if you send the message to your providers SMTP server and it goes through and is not returned to you then Thunderbird has done its job. You need to ask your provider where your mail is going or ask the recipient to check their spam folder to see if it is in there.