This PowerShell script creats an Exchange Online inbound and outbound connector, and adds a transport rulle for the outbound connector.

Add-EXOSmtpGatewayConnectors.ps1

# Name of the on-premises SMTP gateway
$GatewayName = 'NoSpamProxy'

# TLS certificate common name for incoming connection to EOP
$InboundTlsCN = 'smpto.varunagroup.de'

# TLS certificate common name for outgoing connection to EOP
$OutboundTlsCN = 'smpto.varunagroup.de'

# Fully qualified domain name (FQDN) of the on-premises SMTP gateway
$SmartHostFqdn = 'smtpo.varunagroup.de'

# Create OUTBOUND connector from your on-premises SMTP gateway to 
# your Office 365 tenant allowing all SMTP domains for inbound traffic

New-InboundConnector -Name ('Inbound from {0}' -f $GatewayName) ´
-TlsSenderCertificateName $InboundTlsCN ´ 
-RequireTls $true -SenderDomains * ´
-RestrictDomainsToCertificate $true

# Create OUTBOUND connector from your Office 365 to your on-premises SMTP gateway
# for use by a transport rule using a dedicated target smart host FQDN and 
# securing the connection using TLS domain validation

New-OutboundConnector -Enabled $true -IsTransportRuleScoped $true ´
-Name ('Outbound to {0}' -f $GatewayName) ´ 
-TlsSettings DomainValidation ´ 
-TlsDomain $OutboundTlsCN ´
-ConnectorType Partner ´
-SmartHosts $SmartHostFqdn ´ 
-UseMXRecord $false

# Create an EXO transport rule to send all mail message to recipients outside the 
# Exchange organization using the new outbound connector

New-TransportRule -Name ('Outbound Mails to {0}' -f $GatewayName) ´ 
-SentToScope NotInOrganization ´ 
-RouteMessageOutboundConnector ('Outbound to {0}' -f $GatewayName)

# Create an EXO transport role to set the Spam Confidence Level for incoming 
# external messages to -1

New-TransportRule -SetSCL "-1" -FromScope NotInOrganization ´
-Name 'Disable Spam filtering for inbound emails'