Cannot Verify Server Identity. The identity of @outlook.office365.com@ cannot be verified.

this worked.  thanks.

I have two different email accounts.  One "@msn.com" and the other "@outlook.com".  I am having the same issue all are reporting on both OSX and iOS platforms with Apple Mail.  But since my @msn.com has never been affected by this issue, I have to assume this is a Microsoft settings issue.

Others have removed the SSL setting.  I am not very tech savvy with mail applications.  But in doing so, doesn't that diminish email security?

Glad you found a solution that worked for you!  By chance, would you have any thoughts about what to do on the OSX platform?  My settings are already set to outlook.com (I don't use Office 365).  Thanks, and I understand if you don't know as you/other are likely on a PC/Windows platform.

FWIW, after my previous reply (which suggested this was an Apple problem; it was pointed out to me that some users are reporting the same problem on other (i.e., Microsoft) platforms... which I can't address, as I have no devices using MS OS configurations... regardless) I looked into this further, as the problem persisted.

As I am able to send and receive mail, I persuaded myself this had something to do with the certificate in question being related to the use of a web browser (or browsers) on my iPhone and iPad. Speculating that the problem may simply (ha ha) be a corrupted bit of data somewhere in the authentication chain (even a web browser would need to see the certificate if you were retrieving or sending mail via the web on an outlook server), I quit all instances of all web browsers on the devices in question, and rebooted the device. The warnings disappeared.

Further, and I'm sorry if this is redundant information, but you can check on the Trust Certificates installed on your iOS device by going to Settings > General > About and scrolling down to the bottom of the screen where there is a toggle to view the certificates that have been installed.

Tellingly there is no sign of the offending certificate. Also FWIW, the CA of this "rogue" certificate, which is DigiCert, is most certainly on the long list of trusted root authorities that you can look at at this URL on the Apple Support Forums:

List of available trusted root certificates in iOS 12, macOS 10.14, watchOS 5, and tvOS 12

https://support.apple.com/en-us/HT209144

If you are using Windows or some other OS, I'm sure you can find a similar list from those guys.

Jeff IS THE MAN! Did the above and it worked with no issues! My main Office 365 account is nos configured and working on my iPhone. Now trying to figure out the IMAP settings for shared/alias inboxes as those are not working with the same settings.

Agreed this seems to be an IMAP settings problem.  I had 2 Office 365 email accounts synching, one of which was a shared/alias address that used IMAP to connect.  Deleting that IMAP account has stopped the errors.

Adding that shared account to Outlook on a Windows desktop and my iPhone was EASY!  Outlook app for iOS has "add shared mailbox".  Steps are in this article below:

https://support.microsoft.com/en-us/office/open-and-use-a-shared-mailbox-in-outlook-d94a8e9e-21f1-4240-808b-de9c9c088afd?WT.mc\_id=365AdminCSH&ui=en-US&rs=en-US&ad=US

Success with adding the shared account on native Apple Mail on my iPhone:

IMAP ACCOUNT INFORMATION:

Email address: ******@mydomain.org 

(I used to use ******@mydomain.org/sharedboxname)

INCOMING MAIL SERVER:

Host Name outlook.office365.com

User Name ******@mydomain.org

Password is my own password

OUTGOING MAIL SERVER:

SMTP smtp.office365.com

I did not change anything in the SMTP > menu, but here are the settings it defaulted to:

SERVER on

Host Name smtp.office365.com

User Name left blank

Password left blank

USE SSL on

Authentication > nothing selected(!)

SERVER PORT 587