https://forums.veeam.com/veeam-agent-for-windows-f33/smtp-alerts-via-office-365-t58954.html
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_other-mso_o365b/office-365-smtp-authentication-failing-even-with/4e2dcfc4-2626-4c3b-a945-9ff1b3404539?page=2
https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission
$<VariableName> = Get-Content "<text file>"
$<VariableName> | foreach {Set-CASMailbox -Identity $_ -SmtpClientAuthenticationDisabled <$true | $false | $null>}
$Allow = Get-Content "C:\My Documents\Allow SMTP AUTH.txt"
$Allow | foreach {Set-CASMailbox -Identity $_ -SmtpClientAuthenticationDisabled $false}
Get-CASMailbox -Identity <MailboxIdentity> | Format-List SmtpClientAuthenticationDisabled
$Users = Get-CASMailbox -ResultSize unlimited
$Users | where {$_.SmtpClientAuthenticationDisabled -eq $true}
$Users = Get-CASMailbox -ResultSize unlimited
$Users | where {$_.SmtpClientAuthenticationDisabled -eq $false}
$Users = Get-CASMailbox -ResultSize unlimited
$Users | where {$_.SmtpClientAuthenticationDisabled -eq $null}