When running into issues related to Outlook or Exchange Online, you have the option to solve a lot of these issues yourself by using the information in the links shown below:
Contains diagnostics for e.g. :
Migration Exchange Web Services (EWS) throttling policy
Verify that the EWS throttling policy isn't too restrictive for a mailbox data migration that uses third-party tools. (Doesn't apply to Microsoft tools for Hybrid, IMAP, G Suite, or Public Folder migrations.)
Run Tests: EWS Throttling
Mailbox or message size
Check the size of a mailbox or the size of messages (including attachments).
Run Tests: Mailbox Size
Increase or customize Exchange Online mailbox size
Deleted mailbox diagnostics
Check the state of recently deleted mailboxes.
Run Tests: Deleted Mailbox
Delete or restore user mailboxes in Exchange Online
Exchange Remote PowerShell throttling policy
Update the Exchange Remote PowerShell throttling policy for the tenant.
Run Tests: PowerShell Throttling
Email delivery troubleshooter
Check whether email messages are successfully delivered.
Run Tests: Email Delivery
Find and fix email delivery issues
Outlook user connectivity diagnostics
Diagnose Outlook connectivity issues for users by running several checks on possible service-side settings (root causes) so that a tenant admin can quickly fix the issues without involving the users.
Run Tests: Outlook User Connectivity
Fix Outlook connectivity issue
Non-delivery report (NDR) diagnostics
Provide additional information about NDRs that are received through email messages.
Run Tests: Email NDR
Email non-delivery reports in Exchange Online
Archive mailbox diagnostics
Check and identify issues about mailbox archiving.
Run Tests: Archive Mailbox
Enable archive mailboxes in the Microsoft 365 compliance center
Outlook user password prompt diagnostics
Diagnose Outlook password prompt issues for users by running several checks on possible service-side settings (root causes) so that a tenant admin can quickly fix the issues without involving the users.
Run Tests: Outlook Password Prompt
Fix Outlook password prompt issue
Retention policy diagnostics
Check retention policy settings.
Run Tests: Retention Policy
Retention tags and retention policies in Exchange Online
Enable Basic authentication in Exchange Online
Enable Basic authentication for legacy protocols in Exchange Online if previously disabled by Microsoft, or opt out of the disabling process if there is a need to use Basic authentication with specific legacy protocols.
Run Tests: Basic Auth in EXO
Basic Authentication and Exchange Online – June 2021 Update
DomainKeys Identified Mail (DKIM) diagnostics
Validate that DKIM signing is configured correctly and the correct DNS entries have been published.
Run Tests: DKIM
Use DKIM to validate outbound email sent from your custom domain