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I'm running multiple websites on a single server. I'm using sendgrid to send my email.

Right now, I have postfix relay all outgoing mail to a single sendgrid account, using smtp_sasl for authentication.

I need to be able to send each domain's email to a different sendgrid account. So, for, I'd want to relay to sendgrid using one account, and for, I'd need postfix to use a different account to authenticate.

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I don't think so -- they need to go to the same host, but with different authentication. – Joe Van Dyk Jun 13 '11 at 18:55

It's a shame that this one went for so long without a correct answer. And for that matter it's a shame it's not a dealt with by sendgrid's own documentation. The required directives are dealt with succinctly at, which I'll quote here, with some sendgrid specific modifcation:

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

        # Per-sender authentication; see also /etc/postfix/sender_relay.               username1:password1               username2:password2
        # Login information for the default relayhost.
        []              username:password
        # Alternative form:
        # [mail.isp.example]:submission username:password

        # Per-sender provider; see also /etc/postfix/sasl_passwd.               []:submission               []:submission

A few notes here though.

  1. The above switches on the SMTP Envelope From address, not the one in the MIME Header.
  2. Getting postfix to send mail in different directions on the basis of MIME Header checks is more difficult. I'm sure it's possible somehow, but postfix is not designed for this.
  3. DKIM Doesn't care whether either the SMTP Envelope from address orthe MIME Header From address match the domain used for the DKIM signing. It may be that the spam policies of some receiving servers do care. (More info much appreciated here).
  4. Because of 1. and 3., you quite possibly don't need to send to separate sendgrid sub-accounts at all.
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This what what I've been doing for the past couple years, has worked great! – Joe Van Dyk Jun 18 '13 at 22:14

sender_dependent_relayhost_maps and sender_dependent_default_transport_maps are your friends.

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Hm, I'm still a little confused. I don't need to change the host, just the username and passwords. – Joe Van Dyk Jun 13 '11 at 20:55

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