Can't fordward calendar invite with outbound connector in use

[NYDadmin]

Hi,

we're using barracuda for inbound and outbound spam filtering, the issue we're having is that when we forward a calendar invite it doesn't go out via the outbound connector it tries to go out via office 365 default SMTP and therefore get's blocked, see bounce back message below.

Again this is ONLY happening on EXTERNAL FORWARDS of calendar invites how can I correct it?

I already open a ticket with Microsoft support and they simply didn't have an answer so they said that they can't help us because we're using the connector.

Any ideas?

TIA

[#2]

The NDR message seems to be referring to some transport rule, have you run a message trace on O365 and looked at the details? It will show you which rule was triggered.

[#3]

Yes we're using a transport rule for all outbound messages to go through barracuda for outbound spam filtering, but that's the problem here, calendar invites forwards are not going through that transport rule and therefore get blocked.
Why aren't they going out via the transport rule?
How can i make sure that they go out via the transport rule?

TIA