Contact Form Works On One Server But Not Another

[Title de-capitalized; please do not shout at us]

Hi Guys, PLEASE PLEASE PLEASE can someone help me, i’ve been bashing my head for 2 days now and still have no answer although i think i know the issue: Basically, all of a sudden none of my contact forms send the email, i have lots of websites using a few different contact form plugins, all working before but now not. After trying everything i tried moving the website to another of my servers and every website’s form now works, so basically 2 identical websites and contact forms, one works fine on one server and i receive the email from the form, on the other server the form submits but no email is received. 1&1 internet say it’s the plugin, but it’s obviously not as it works perfect on another server. To summarise i have tested 3 different websites using 4 different contact forms and they ALL work, once copied to another server, the websites are fine and forms submit, but NO emails are received, is it the server and if so what should i tell the guys at 1&1 as they keep saying its the plugin… REALLY APPRECIATE ANY ANSWERS AS THIS IS DRIVING ME INSANE ??