Contact forms stopped working suddenly

My client’s site: https://www.solarbarn.co.uk. Hosted at Force36 in the UK. We had Contact Form 7 and it was working fine but suddenly stopped sending emails. The form says the mail is sent successfully but they never arrive. Replaced Form 7 with the Jetpack form and same thing. Says they’re sent but they don’t arrive. I’ve updated everything possible on the site, all the plugins etc, but still not working.

Now completely at a loss where to go next. Any ideas? Thankyou!