As wilt said, what you're getting is a false-positive. The logic which processes the data already sanitizes the content before sending the emails, meaning that any attempt to insert <script> tags will be thwarted. You can see it just by trying it yourself ... you'll see that the < and > symbols are stripped and replaced when entered in the contactname field, and the email field data is rejected since it doesn't actually contain an email address of the correct form. Making the change suggested by wilt will provide a slight amount of additional visual comfort, but is not strictly necessary since there are additional sanitizations that catch the problem.
That said, the change wilt suggested is incorporated into both the upcoming v1.3.9 and v2.0.0 releases.
In the meantime, report the false positive to your scanning company, and if they don't listen to you, get them to prove the vulnerability manually and supply the exact details of actual risk ... because what you've shared thus far is not a real risk.
And, if you do have a real risk, you need to report it in the Security Concerns section of the forum, not the Bug Reports area.