Do you get any errors back when you e-mail the mailbox ?

I do not receive any errors per se.   I did a message trace and saw the fails based on the non authenticated user. 

Do you forward e-mail to Fresh or use custom mailbox ?

I have tried both ways.  Forwarding to the support address in Fresh and using the fresh address to send to

And the mailbox have the permission to to send forward external ?

yes

If you open up the shared mailbox in OWA and forward a e-mail manually, what happens ? ( you need send as access to the mailbox) 

goes through with no issue

Hi.

Apologies for jumping in, but just trying to send some additional thoughts:

Is there any Outgoing Anti-Spam | Relay | Smarthost or are outgoing emails sent directly to Internet? 

Anyway, what you are experiencing is a known | standard behavior in mail systems.

Just curious about “Forwarding to the support address in Fresh and using the fresh address to send to”: Do you currently have a dedicated mailbox with your support email address in which exists a rule to send/forward emails to the FreshService Email Address (emailaddressonyourdomaininstance@*.freshservice.com).

In case you are defining a DL with your support address and include de FS, it won’t work. You need to explicitly use a dedicated mailbox for this to work.

But both these previous statements to check are valid when the email is actually attempting to go out from your instance. You also mentioned that in the message trace, you see a fail because of unauthenticated users. So, this leads me to think that emails aren’t still being sent out; there haven’t actually been processed but EO, as it seems they may be rejected at connection level.

Would you be able to share more details or the exact message / log you’re seeing in the Message trace in order to confirm (or reject) this hypothesis?

Depending on what you are actually getting, the approach may be: setting up a relay within your mail infra, in which you allow your specific hosts from which you are trying to send emails from, and set them to: either allow to send unauthenticated emails, or, to allow authentication in Defender and the other Vendors solutions you are working with.

Authentication may be in the form of Token, passwords, or may even be in your SPF record, which might need some adjustment to allow your external parties (in case they are cloud-based) to send emails from your domain. That way, they will become (SPF-)authenticated.

These is what I have usually found in different customers with different solutions when attempting to do that: Forward to external services emails from other Notifications solutions.

Regards,