Postfix as a forwarder: how to configure it as securely as possible?

Question

postfix

I use Postfix only for forwarding mail from non-existent "boxes" like [email protected] to [email protected] On your VPS.
To do this, set up aliases in the file:
/etc/postfix/virtual
[email protected] [email protected]
And set up the file (4 lines): main.cf
virtual_alias_domains = mydomain.com
virtual_alias_maps = hash:/etc/postfix/virtual
smtp_tls_security_level = may
smtp_tls_loglevel = 1
Everything works (although mail does not get through from all domains, but gmail is probably filtering here ...).
Question: what to do to close all the holes (I'm not an admin, but it's obvious that I didn't take into account something).
Thanks