Some organisations, in particular German ones, require encryption between your Exchange Online mail servers and their mail servers. This can be enforced by adding their domains to an outbound TLS connector in Office 365.

In my case, this had to be enforced (the default is oppertunistic). If you get a long list, you’ll want to script this with the following snippet. Remember to make sure the first line of your CSV has the header/colum name ‘domain’ 🙂

#Author: Jos Lieben
#Help: www.lieben.nu
$UserCredential = Get-Credential $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session -AllowClobber -DisableNameChecking

$connector = Read-Host "Type the name you wish to give this connector"
$csv = Read-Host "Type the path to the file with TLS domains"

$domains = Import-CSV -LiteralPath $csv

$newDomains = @()
foreach($domain in $domains){
$ND = [String]$domain.domain
$ND = $ND.Trim()
$newDomains += $ND
}

try{
New-OutboundConnector -Name $connector -recipientdomains $newDomains -ConnectorType "Partner" -TlsSettings "CertificateValidation" -ErrorAction Stop
}catch{
Write-Error "Failed to modify outbound connector $connector $($Error[0])"
}