Adding to the advice already given, I am getting frustrated with Zen's legacy email service for home users.
It's basically a POP service and the servers are occasionally getting blocked by Microsoft for their incoming email service.
https://servicealerts.zen.co.uk/alert/11226/
I also cannot get their email service to function on the Outlook App despite following the guidelines offered for using POP servers
I wouldn't be surprised if email for home users would eventually be withdrawn for all as other ISPs have previously enacted.
The recommendation is to always keep your email service separate from your ISP.
Zen Unlimited Fibre 1 - 27/6.5