I use IMAP with Office 365, and I am getting "Retrieval using the IMAP4 protocol" messages. What are these?

This FAQ was last modified on: Thursday, December 21, 2017 08:42am
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This is where your IMAP calendar has "subscribed" to your Calendar folder.  The Calendar folder is only intended to be read by Microsoft clients.  To stop seeing these messages, please adjust the IMAP Folder Subscription settings on your IMAP email client.

Example:

example showing a subscribed calendar folder

 

Additional Resource:

"You can’t connect to your contacts, tasks, or calendar when you connect to your account using POP3 or IMAP4."

https://support.office.com/en-us/article/Use-POP-or-IMAP-to-connect-to-Office-365-for-business-or-Microsoft-Exchange-accounts-44F951CC-2041-47ED-B674-506889CA9D8B