IMAP-mail

You must have installed it unless you have an administered company phone.

Find the app and delete it is the usual method.

If your phone is company administered get the admin to take it off or change the whitelist.

Digicert? So you are running Antivirus/security app.

Please uninstall ASAP!!

Subsequently this turned out to be a global Microsoft mess-up with digital certificates

Depends on what the alert said.

What kind of alert?

Normally to get rid of the alert, you address the issue the alert is referring to. Since you have provided any sort of details, its impossible to provide any advice.