>>Hi,
>>
>>Since this morning, my Outlook does not connect to Gmail accounts.
>>
>>Currently my Gmail emails (in Outlook) are set to use POP/SMTP. Do I need/have to change it to IMAP?
>>
>>TIA
>
>Just for testing, in my MS 365 Outlook, I added an email account (myname@gmail.com). I set it up as IMAP.
>It seems to work since I sent myself a test message from another account and receive it in Outlook.
>However, I did not receive this message on my iPhone. Why?
>
>The difference, so far, I see between the POP and IMAP accounts is that the new IMAP account stores messages to a separate file, myname@gmail.com.ost
>This is, unlike before, when I had the email account as POP, the messages were saved in .pst file.
>The first conclusion, is that I have to back up more file.

Outlook .ost files are cache files, not formal stores. They're used when the primary message store is a remote server, either Exchange or IMAP. Outlook syncs .ost files with their respective server mailboxes. If synced, and you want to look at a mail, calendar or other item, Outlook will retrieve it from the .ost cache file rather than going to the back-end server. This is more responsive and reduces network traffic.

https://www.howto-outlook.com/howto/backupandrestore.htm#restore-ost

You do need to be careful with IMAP. Outlook lets you create items of various types such as mail, calendar, tasks, notes etc. It may not be obvious which of those types are supported by your IMAP server. Any that aren't are present only in your .ost file. Rather than backing up the .ost file, within Outlook you should instead periodically export everything to a local .pst data store.

Also with IMAP, be alert to any reported sync errors. Address them right away, don't let them fester, otherwise different devices will show different items.

>Question:
>If I delete this message, in Outook, will it also delete it on the Google server?

As soon as Outlook syncs a change with Gmail, any such change is reflected at Gmail.

>UPDATE/ANOTHER QUESTION: I asked the wife to send me a test message to my email myname@gmail.com.
>But I don't get it on iPhone. I think iPhone is set to connect to the gmail server using POP3. Is this the reason I don't see the test message on iPhone? Do I need to change how iPhone connects to Gmail and change it from POP3 to IMAP?

Don't mix access types to a single mailbox. Either use IMAP for all devices, or POP3.

If you need or want to get the same view of a mailbox across multiple devices, don't use POP3. You can get a half-assed "sync" for incoming mail using the "Leave mail on server for XX days" feature, but that doesn't work for sent mail or other Outlook items. IMAP is better.

If you need or want to use Outlook, and you need to support multiple devices, the best option is an Exchange back end.