SMTP throttling

TLS just enables encryption on the smtp session and doesn't directly affect whether or not Postfix will be allowed to relay a message.

The relaying denied message occurs because the smtpd_recipient_restrictions rules was not matched. One of those conditions must be fulfilled to allow the message to go through:

smtpd_recipient_restrictions =
    permit_sasl_authenticated
    check_recipient_access hash:/etc/postfix/filtered_domains
    permit_mynetworks
    reject_unauth_destination

To explain those rules:

permit_sasl_authenticated

permits authenticated senders through SASL. This will be necessary to authenticate users outside of your network which are normally blocked.

check_recipient_access

This will cause postfix to look in /etc/postfix/filtered_domains for rules based on the recipient address. (Judging by the file name on the file name, it is probably just blocking specific domains... Check to see if gmail.com is listed in there?)

permit_mynetworks

This will permit hosts by IP address that match IP ranges specified in $mynetworks. In the main.cf you posted, $mynetworks was set to 127.0.0.1, so it will only relay emails generated by the server itself.

Based on that configuration, your mail client will need to use SMTP Authentication before being allowed to relay messages. I'm not sure what database SASL is using. That is specified in /usr/lib/sasl2/smtpd.conf Presumably it also uses the same database as your virtual mailboxes, so you should be able enable SMTP authentication in your mail client and be all set.

IIS7 SMTP is the same as IIS6 SMTP. It wasn't updated, so even the management is done with IIS 6 Manager (even though everything else is IIS7). Edit the server properties and make sure that 127.0.0.1 is added as an approved relay IP address. That will allow it to work without authenticating as long as it's sent from the local machine.

Note: That was required with IIS6 too though so possibly you did this with the IIS6 server at some point in the past.

That's the only change I make to a new IIS SMTP server to ensure that it works from the local machine. (not counting spam settings like reverse and forward DNS entries)