[AuckLUG] Spam emails

[Stuart R Mealor]

Hi
This isn't really a specific Linux issue, but I wonder if anyone here
can offer advice...?

We have a domain (hosted on our Ubuntu server with static IP assigned by
Telecom), but our email is all via Xtra (I didn't want the overhead of
maintaining a mail server at this time).
We just download email to Thunderbird from our domains imap
We use smtpr.xtra.co.nz (port 465) for sending.

I have noticed an increase in 'Undeliverable email' hitting our email
admin account from:
Postmaster...
Mail Delivery System...
Mail Delivery Subsystem
Mailer-Daemon...
etc...

In all these cases, it looks like someone has just added a 'random' name
to our domain, and sent it off to other email address.
Are these emails really going to other email addresses and looking like
spam sent by us?
Is there anything we can proactively DO about this?
Is this a result of our email accounts being on a Telecom server?

Advice or knowledgeable comments(!) gratefully received.
Stuart