IMAP and Exchange calendar sync issues

Hi Steve Burns,

Welcome to Microsoft Q&A forum. 

Sorry for the inconvenience—especially since you’re trying to keep the two calendars separated by account.

Based on your description and the fact you are using Classic Outlook, this behavior can occur by design when an IMAP (Yahoo) account is used alongside an Exchange/Microsoft 365 account in the same Outlook setup. In Outlook, meeting requests are typically saved to the calendar of the account that received the email, with one important exception: if the receiving account has no associated calendar folder, the meeting request may be saved to the default calendar instead. This is commonly seen with secondary IMAP accounts, which can cause Yahoo invites to appear on the Exchange calendar when Exchange is set as the default store/calendar.

To keep the two calendars separated, here are the most relevant options in Classic Outlook:

1. Use separate Outlook profiles

• Create one Outlook profile for the Yahoo (IMAP) account and a separate profile for the Exchange account.

2. Check / adjust the Default Data File

• Go to File > Account Settings > Account Settings > Data Files and confirm which data file is set as Default.

Important note: Copying meeting items between calendars is not recommended because Outlook can break the link between the copied meeting and the original, and newer Outlook builds may restrict copying meetings.

Please feel free to let me know if you have any further updates, thanks.

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".  

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread. 

Outlook treats Yahoo as an IMAP account and the Microsoft 365 mailbox as an Exchange account. IMAP accounts in Outlook sync only mail folders; calendar and contacts for those accounts are local to the Outlook profile and are not true server-side calendars like Exchange. Because of this, meeting invitations sent to the Yahoo (IMAP) address can be delivered into the Exchange mailbox and end up associated with the Exchange calendar, especially when the Exchange account is set as the default data file or default calendar.

From the provided information, only the following supported behaviors are clear:

• Exchange accounts sync mail, calendar, and contacts with the server and other clients (for example, Outlook for Windows and Outlook for Mac) as a single, integrated mailbox and calendar.
• IMAP accounts sync only email; calendar data is managed differently and does not behave like an Exchange calendar.

There is no documented method in the context to force Outlook to keep meeting requests for an IMAP account strictly bound to a separate IMAP calendar in the way Exchange does, nor a supported way (in the provided material) to re-route those meeting items automatically between calendars.

Given the constraints in the context, the only fully supported guidance is:

• Use the Exchange calendar as the primary calendar for meeting requests and sharing, since it is the one that syncs reliably across devices and Outlook clients.
• Treat the IMAP account primarily as an email-only account; its calendar cannot be managed or synced in the same way as the Exchange calendar.

References:

• Sync basics - what you can and cannot sync