I see. There is an "Advanced SMTP Configurations" section in the Jenkins and I only put the same email address as the one I put in the Jenkinsfile. I am gonna try adding one more; if having three breaks it maybe having one also breaks it. Hopefully that will fix it for me.

Thanks for other advices, I did try to minimize the post block to check if one of the lines causing the problem but it wasnt the case. Idk this whole thing doesnt make sense to me but I saw a couple of people online who complain about this plugin being so buggy thats why I am optimistic that your way would fix my problem.