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Postfix Configure Multiple ISP Client SMTP Authentication

How do I support multiple ISP accounts (smarthost) in the Postfix SMTP client for relaying email? For example:

[a] user1@isp1.com must be relay via smtp.isp1.com.
[b] user1@isp2.com must be relay via smtp.isp2.com.
[c] me@mycorp.com must relay all my default email via smtp.mycorp.com.

Only Postfix version v2.3+ supports multiple ISP accounts via sender-dependent override for the global relayhost parameter settings. Open your main.cf file:
# vi /etc/postfix/main.cf
Add or modify the settings as follows:

smtp_sender_dependent_authentication = yes
sender_dependent_relayhost_maps = hash:/etc/postfix/relayhost_map
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/passwd
relayhost = [smtp.mycorp.com]:submission

Create /etc/postfix/relayhost_map file:
# vi /etc/postfix/relayhost_map
Append configuration as follows (please note that default smtp.mycorp.com is not included) :

# Per-sender provider
user1@isp1.com		[smtp.isp1.com]
user1@isp2.com		[smtp.isp2.com]

Create update /etc/postfix/passwd file as follows:

# Per-sender authentication
user1@isp1.com		user1@isp1.com:password4isp1
user1@isp2.com		user2@isp2.com:password4isp2
# Login for the default relayhost
[smtp.mycorp.com]	defaultUsername:defaultPassword

Save and close the file. Restart Postfix:
# postmap hash:/etc/postfix/passwd
# postmap hash:/etc/postfix/relayhost_map
# /etc/init.d/postfix reload


  • smtp_sender_dependent_authentication = yes : Enable sender-dependent authentication in the Postfix SMTP client; this is available only with SASL authentication.
  • sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay : A sender-dependent override for the global relayhost parameter setting. The tables are searched by the envelope sender address and @domain. A lookup result of DUNNO terminates the search without overriding the global relayhost parameter setting.
  • smtp_sasl_auth_enable = yes : Enable SASL authentication in the Postfix SMTP client. By default, the Postfix SMTP client uses no authentication.
  • smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd : SMTP client lookup tables with one username:password entry per remote hostname or domain, or sender address when sender-dependent authentication is enabled.
  • relayhost = [mail.myisp.net]:submission : Setup default relay host.
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{ 7 comments… add one }
  • Sureshkumar March 18, 2009, 1:38 pm

    Previous I was tried this concept but it was not working
    My scenarios adding multiple users with SMTP auth
    With username and passwd
    Am waiting for your reply.please scrap me or mail me
    this is my email id : krisuresh001@gmail.com

  • Sachin September 4, 2009, 6:50 pm

    Is this also supported on sendmail?

  • Coschuetzer October 11, 2009, 11:40 pm

    At first, thanks aslot for making this (almost) clear.

    What I don’t understand is: If …
    “[a] user1@isp1.com must be relay via smtp.isp1.com.
    [b] user1@isp2.com must be relay via smtp.isp2.com.”
    (so there is only user1 involved)

    … then why is this line in /etc/postfix/passwd ?
    “user1@isp2.com user2@isp2.com:password4isp2″

    Where does user2 coming from all the sudden?!

    Shouldn’t it be like this
    “user1@isp2.com user1@isp2.com:password4isp2″

    or do you just want to show, that the username on the “real” isp2.com has not necessarily to correspondent with the “local” user1?

    Thanks for answering,

  • Wolfgang muller November 3, 2009, 5:15 pm

    Global internet providers aim is to provide access that is effortless to utilize. They provide access that is as fast as you absolutely need it to be, and will work with the system you have.

  • David Perez February 5, 2013, 8:04 am

    This configuration works well for the relay of all emails in each domain. My question now is whether there is a Postfix configuration

    allowing the relay emails sent only to ‘hotmail.com’ for each domain. That would solve my problem, because the only emails I am having

    reception problems are with ‘hotmail.com’, others are working properly and I can send to my VPS server.

  • Tomas August 16, 2013, 2:27 pm

    But, how to use a variable in /etc/postfix/passwd???
    I want postfix to authenticate with my ISP using the sender email addres as login and a fixed password. For example

    user1@isp.com user1@isp.com:alwayssamepass
    user2@isp.com user2@isp.com:alwayssamepass
    user3@isp.com user3@isp.com:alwayssamepass

    But I don’t want to write thousands of users. I want something like:
    *@isp.com $sender_address:alwayssamepass

    Do you understand mem?

  • Mikko Ohtamaa May 6, 2015, 12:30 pm

    Thank you, excellent guide!

    I had the same issue that one website on a shared host had to use office365.com SMTP server. I was about to cry when reading the official Postfix documentation – it would take forever to process all the information there. With this guide I was able to solve the problem in a matter of an hour.

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