Login to Microsoft 365 Defender: Policies & rules --> Threat policies --> Anti-spam policies
https://security.microsoft.com/antispam

Edit the "Connection filter policy (Default)" or make a new rule to allow the IP address(s).

An example error:
Message Delivery Failure. Attempt (0): Could not connect to mail server for domain. The remote mail server could not be contacted at this time. Message has been requeued.

Keywords: Email, E-Mail, 365, Hosted, Whitelist, Allow

HealthChecker.ps1
https://microsoft.github.io/CSS-Exchange/Diagnostics/HealthChecker/

This free utility isn't easy to use, but can backup and restore data to/from gmail accounts, and preserves labels. It can restore MBOX files taken from Google Takeout.
https://github.com/GAM-team/got-your-back


You can also use Thunderbird or Outlook to set up two accounts, and manually backup all folders.

https://takeout.google.com/settings/takeout


Keywords: Google Mail, Gmail, Backup, Restore, Sync, Clone, Archive, Thunderbird

These steps let you add free accounts (without email access) that you can set as Administrator.
https://knowledge.workspace.google.com/kb/how-to-create-a-super-admin-user-without-a-google-workspace-subscription-000009398

Keywords: Gmail, Google Workspace, Free, Superuser, admin, license

Google Admin --> Apps --> Google Workspace --> Gmail --> Compliance --> Content Compliance
https://www.goldyarora.com/selective-email-forwarding/



Keywords: Forward, Delete, Redirect, Gmail, Google Apps, Email, Block

https://www.hmailserver.com/functionality

1: Change domain to "Internal relay"
2: Create Distribution Group with all accounts in it (or use Dynamic Group).
3: Create mailbox (probably a Shared mailbox) for catchall.
4: Create a rule to move all messages to catchall account if the recipient is NOT a part of above group.
https://www.godaddy.com/help/catch-all-email-not-supported-with-microsoft-365-40130


keywords: catch-all

It is best to do this over a long weekend, if possible.
Make a backup of Exchange first.
Export Public Folders to PST
Start migration using Microsoft 365, from Exchange Admin, use Migration feature.





Keywords: Office 365, Exchange Online

Migrate to Microsoft:
https://docs.microsoft.com/en-us/exchange/mailbox-migration/migrating-imap-mailboxes/migrate-g-suite-mailboxes
https://docs.microsoft.com/en-us/exchange/mailbox-migration/migrating-imap-mailboxes/prepare-gmail-or-g-suite-accounts



Keywords: Google Gmail, Google Apps, Microsoft Exchange Online, Hosted Exchange, Outlook

You need to enable SMTP sending from Microsoft 365 Admin Center
Go to User list, click on a user. Click on Mail tab, then Manage email apps, then check Authenticated SMTP.
https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission


Keywords: SMTP, Send Mail, Email, Authentication, OATH2, Thunderbird

Goto yahoo.com
Login to the account, go to Account Info. It should redirect you to AT&T.
Scroll down to "Manage secure mail key", create a special password to use for IMAP/POP3