Upload S/MIME user certificate with outlook to the GAL

Made some Tests in the Past. Maybe these Commands help you further

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#Set UserCertificate in EXO
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Connect-ExchangeOnline
$cert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2("D:\temp\a.bohren@icewolf.ch_swisssign.cer")
$certArray = New-Object System.Collections.ArrayList
$certArray.Insert(0,$cert.GetRawCertData())
Set-Mailbox -Identity email address removed for privacy reasons -UserCertificate $certArray

$MBX = Get-RemoteMailbox -Identity email address removed for privacy reasons
$Cert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($mbx.UserCertificate)
$Cert | fL

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#AD
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$ADUser = Get-ADUser -Identity "a.bohren" -Properties userCertificate
$ADUser.userCertificate
$Cert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($ADUser.userCertificate)
$Cert | fL
Set-ADUser -Identity "a.bohren" -Clear "userCertificate"

$cert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2("D:\temp\a.bohren@icewolf.ch_swisssign.cer")
Set-ADUser -Identity "a.bohren" -add @{"userCertificate"=$Cert.Rawdata}