After mailbox migration to new Exchange server or Exchange Online, Outlook refused to send emails. Outlook displays “Error 0x800401F” upon send/receive.
Outlook profile and/or OST/Offline Address Book is corrupted. Or the Outlook profile has some legacy settings which is not compatible with new Exchange server (especially Exchange Online in Office 365)
Delete Outlook Profile and Outlook OST and other files and recreate new Outlook profile. Follow the instruction below (copy/paste & send it to user).
1. Save all the emails in Outlook as Drafts (or move to Drafts folder)
2. Close Outlook
3. Open this location at File Explorer: at C:\Users\<username>\AppData\Local\Microsoft\Outlook
4. Delete all files at above location
5. Go to Control Panel and Open Mail (Microsoft Outlook 2016) (32-bit)
6. Click Data Files… and note down any personal PST files and file locations. Click Close.
7. Click Show Profiles…
8. Delete the listed profile (select the profile and click Remove)
9. Click Add button to add a new profile. Provide your email address and password and following instructions.
10. Add the PST files back if any noted down on step 5.
11. Close all dialog boxes. Open Outlook.
After the above steps, user have to manually reassign the mail Signatures in Outlook Settings.