edit server in iphone exchange email
Generally speaking all exchange servers since (I believe) 2007 have an automatic configure function. So if your mail server is at mail.company.com the automatic configure URL will be autodiscover.company.com.

Assuming your mail server was set up correctly along with the correct DNS entries, merely removing and re-adding that account from your iPhone should fix the issue. Assuming your IT department hasn't done something non-standard.