"www.stanza.co cannot be verified" pop-up on iPhone

Best would be to Delete and Add the email account that is creating the issue.

You may also try changing SSL settings

Settings > Password & Accounts > [Select the Mail account} > [Tape on Account with the email ID} > Advanced > SSL Settings > [Disable]

Click <Account (in Blue)

Click on DONE (in Blue) when exiting in the previous screen

To improve understanding of the phenomenon, it would be highly beneficial if you could share a screenshot. Visual representations carry greater impact than written descriptions.

Please remember that this is a public forum, so when attaching a screenshot, please avoid including any personal credentials such as IP addresses, card details, email IDs, Apple IDs, IMEI numbers, serial numbers, phone numbers, order IDs, or any identifiable location information if you are sharing a map.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.

Check for subscribed calendars (possibly a sports team)

Delete spam calendars and events on iPhone - Apple Support

How can I stop this?

Delete the subscription to a sports team calendar.