Thanks for posting your question in the Microsoft Q&A forum.
If you don't want to manually add a new domain name alias for each user, we recommend that you use the Exchange Online PowerShell script.
1.Connect to Exchange Online
Connect-ExchangeOnline -UserPrincipalName youradmin@yourdomain.com
2.Get all user mailboxes:
$mailboxes = Get-Mailbox -ResultSize Unlimited
3.Loop through each mailbox and add the new aliases:
foreach ($mailbox in $mailboxes)
{
$newAlias1 = $mailbox.Alias + '@newdomain1.com'
$newAlias2 = $mailbox.Alias + '@newdomain2.com'
Set-Mailbox -Identity $mailbox.Identity -EmailAddresses @{add=$newAlias1,$newAlias2}
}
Below is my test, I only make changes to two users.
Checked the user and found that the alias had been added
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