Postfix email server not receiving email from external email

You have:

smtpd_sender_restrictions = reject_unknown_sender_domain

smtpd_recipient_restrictions = 
    reject_unknown_sender_domain, 
    reject_unknown_recipient_domain,
    reject_unauth_pipelining,
    permit_mynetworks, 
    permit_sasl_authenticated, 
    reject_unauth_destination

As these are missing the final permit in the end, only mynetworks and SASL authenticated are allowed to use the server. This limitation affects all mail including the messages having this server as the final destination. This gives the fatal: no SASL authentication mechanisms error.

If you wish to have different settings for SASL authenticated users, enable submission. That's another instance of SMTPd listening on TCP/587 for the outbound mail of your own users. Following the examples in Postfix and Dovecot SASL HowTo you'll configure your Postfix exactly that way (see the example in Using SASL with Postfix submission port).