emails are sent to mailserver but get marked as ” read” right away?

Hi!
I have been working on a website from a friend. I’m a WP beginner, so I only use some simple plugins. My “area of expertise” is marketing and online advertising and I have set up some advertising campaigns for them.
I’ve recently added a facebook tracking pixel to CF7 to keep track of conversions. So far so good.
Only, i’ve just heard that emails don’t get delivered the same way they used to, and it started around the time I made some changes in their wp site.

The situation is that emails do get sent, and arrive at the server, but are then immediatly marked as “read”. Since my friend uses POP to send emails to a device or pc, the emails don’t get “pushed” to his device. He thought it was weird when he didn’t received any emails for a few days.

He can access and read them via webmail, but for them this means that they probably missed a few bookings (they own a hotel) because of this.

My first thought was: I can’t have created this error, can I?

With my basic knowledge of servers, I think that since the emails do arrive at the server, the problem lies withe the server, or with the settings on his device. And not with CF7.

But just to be sure: is there anything I cloud have done that has created this situation?

The only things I have done: activate the plugin “insert headers & footers” (to add analytics and facebook tracking code to each page) and have added a tracking pixel to the submission button in CF7.

I haven’t touched the email server or settings.

I hope someone can tell me what the problem might be!

Thanks in advance.