The following is a breakdown of Microsoft Support calls (28) I've made in the last few years:

Date	Issue	Resolution
Jul 2018	Exchange servers bluescreening.	Windows firewall was enabled for all outgoing connections, this was removed.
Aug 2018	Single Exchange server crashing.	Emulex FC Driver "elxfc.sys" needed to be updated.
Nov 2018	Exchange servers not responding to network requests.	Caused by the server anti-virus, Trend Micro.
Mar 2019	Exchange servers dropping connections for Outlook clients.	Not enough memory on active servers.
May 2019	No mailbox to select when migrating to O365.	Azure AD sync had not been updated since Exchange was installed in environment.
Jun 2019	Random mails not sent via Outlook with error message.	User was saving mails to drafts before other user tried to send them.
Jul 2019	Hybrid Connection Wizard (HCW) gives error.	Manually configured and ignored errror.
Sep 2019	Advisory on O365 mail-flow	Advice given.
Oct 2019	Image in email from Notes gave warning.	Set the image size and this resolved the error.
Feb 2020	Mail-flow to shared mailboxes not working.	Shared mailboxes were not synced to Azure AD correctly.
Feb 2020	Teams calendar not showing.	Enabled OAuth in environment.
Feb 2020	Group and Mail Contact errors.	Enabled AAD writeback.
Feb 2020	Exchange update broke IIS.	Fixed IIS directories.
Mar 2020	Hybrid setup with NetScaler failing.	Removed SSL offloading, used SSL bridge mode instead.
Apr 2020	Outlook not using OAuth.	Bypassed web proxy.
Jul 2020	Outlook365 slow to open for on-prem users.	Registry key supplied but not used.
Nov 2020	Mails to O365 groups fail.	Groups did not have onmicrosoft.com addresses.
Nov 2020	Mails deleted from system.	No resolution.
Nov 2020	User could not logon to webmail in O365.	Fixed AAD sync errors for object.
Apr 2021	DLP not working for mailbox.	By design, mailbox was on-prem not O365.
Apr 2021	Teams photo disappears.	Photo took a long time to sync correctly.
Jun 2021	Intermittant mail failures.	Issue with 3rd party certificate.
Jun 2021	Free/busy not working on-prem to O365.	Changed URLs.
Nov 2021	Outlook Mobile setup not working.	Firewall rues needed to be updated for HCW.
Jun 2022	On-prem delegated user can't release email.	Design change request accepted.
Sep 2022	Centralized mail flow getting quarantined.	Mail filter removed.
Oct 2022	Outlook disconnects on-prem.	Removed NetScaler.
Dec 2022	Teams calendar missing after mail migration to O365.	Issue resolved itself after a day.