Webform Including Site Admin Email Address in Email Headers

I have built a site for a client using webform 7.4.12. I've built many sites, but this is the first with Webform 7.4 and I've never run into this issue before. The webforms are set up with the client email address as the default, and each of 3 forms have the default set as the From address on the forms. When the client receives an email from the site my email address, the admin email address appears in the headers of the email. This what they see at the top of emails from each of the forms:

[site admin email] on behalf of [email form 'Name' component] via [site name] [webform default email]

I hope this makes sense. This site is not hosted on my server, it's hosted at Bell Hosting so I'm not sure if there's something in their configuration that could be causing this but I think it's unlikely because the site admin email address should not be appearing anywhere. It should never be sent by the form.

I've attached a screen capture of the email header as it's displayed in Outlook.