Sending email through multiple external SMTP or local MTA

[Frank]

Hi,

We are using 2 external services to send emails: Sparkpost and Sendgrid. Mails are channeled through one or the other according to sender (using this parameter in main.cf):

smtp_sender_dependent_authentication = yes

This setup works fine, since both services require the same exact SASL setup (also in main.cf):

smtp_sender_dependent_authentication = yes
sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt
header_size_limit = 4096000

Two questions:

1. How could we use the local MTA to send some of the emails (sender dependant)? The local postfix installation cannot require SASL, or use passwords.

2. Is it possible to use the local MTA as default sending transport, and use an external service according to the recipient MTA domain? Our local box cannot send to any Outlook/Live/Hotmail domains due to IP restrictions: all mails to these domains need to be sent through either Sparkpost or Sendgrid.

Any help really appreciated.