我怎么能等待一个线程完成运行？Thread.join让我的主线程没有响应？

I'm working on project which sends mails to multiple users using windows application,But what is happening is,when I use Thread.Join() Method it is making my Main UI non-responsive...... What I want is.... I want to run SMTPClienTSetting() method on seperate thread and want my Main UI to remain responsive for other work..I wnat to run this mail sending acvtivity at the background...Please suggest me using threading concept only.....I'm working on project which sends mails to mul