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I'm trying to get email reports from our AWS EC2 instances. We're using Exchange Online (part of Microsoft Online Services). I've setup a user account specifically for SMTP relaying, and I've setup Postfix to meet all the requirements to relay messages through this server. However, Exchange Online's SMTP server will reject messages unless the From address exactly matches the authentication address (the error message is 550 5.7.1 Client does not have permissions to send as this sender).

With careful configuration, I can setup my services to send as this user. But I'm not a huge fan of being careful - I'd rather have postfix force the issue. Is there a way to do this?

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4 Answers

In order to rewrite FROM addresses you should use smtp_generic_maps.

According to postfix documentation :

/etc/postfix/main.cf:
    smtp_generic_maps = hash:/etc/postfix/generic

/etc/postfix/generic:
    his@localdomain.local       hisaccount@hisisp.example
    her@localdomain.local       heraccount@herisp.example
    @localdomain.local      hisaccount+local@hisisp.example

Then do:

sudo postmap /etc/postfix/generic
sudo /etc/init.d/postfix reload

References:

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You can use smtpd_sender_login_maps to specify a list of maps: sender address - user.

Example:

smtpd_sender_login_maps = 
    hash:/etc/postfix/login-map 

/etc/postfix/login-map:

mail1@domain    userlogin
mail2@domain    userlogin, otheruser@example.com

It does work for sending, it should work for relaying the same way.

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Is the purpose to rewrite the from address at queue time? My impression is that smtpd_sender_login_maps is used to reject senders (as part of smtpd_sender_restrictions), not to translate logins to email addresses. With this in place, echo "This is a test" | /usr/sbin/sendmail test@example.com still queues (and eventually relays) the mail as coming from user@domU-DE-AD-BE-EF-00-01.compute-1.internal –  John Whitlock Jun 3 '10 at 21:16
    
My bad, I somehow understood you do authorize to your postfix. In such case smtpd_sender_login_maps would limit what user can use what sender adddress. –  silk Jun 4 '10 at 11:32
    
This may end up being part of my end solution. I could reject all but the one sender address on the front end, and notify the postmaster of any front end rejections, to let me know when a service isn't configured correctly yet. I'm looking at smtp_generic_maps, to see if I can use them to modify all local accounts to the One True From Address. But, I need to re-read the postfix docs to understand how it determines local accounts - AWS DNS isn't matching my intuition. –  John Whitlock Jun 4 '10 at 15:41
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Update: On the advice of an IT friend, I'm running postfix on all my servers, rather than making one cloud mail server. Here's my solution so far:

/etc/postfix/main.cf

# output of hostname -f - mail from local users appears to come from here
myhostname = domU-01-02-03-04-05-06.compute-1.internal
# Local delivery - include all 127.0.0.1 aliases from /etc/hosts
mydestination = $myhostname, $mydomain, rest_of_entries_from_hosts
# Needed for address translation to work
myorigin = $mydomain

# Talking to MS Online
# :submission = port 587
relayhost = [smtp.mail.microsoftonline.com]:submission
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =   # Yes, leave empty
smtp_tls_security_level = encrypt
smtp_generic_maps = hash:/etc/postfix/generic

# Enable if you need debugging, but it does leak credentials to the log
#debug_peer_level = 2
#debug_peer_list = smtp.mail.microsoftonline.com

# Only listen on the local interfaces (not the public)
inet_interfaces = localhost

# I left out a bunch of CentOS defaults.  postconf -n is your friend.
# These are included
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases

/etc/postfix/sasl_passwd

# Run postmap /etc/postfix/sasl_passwd after editing
# Also, chown root:root; chmod 600
smtp.mail.microsoftonline.com relayer@hosteddomain.com:YourP@ssw0rd

/etc/postfix/generic

# Run postmap /etc/postfix/generic
# I've seen local mail come from either source
# output of dnsdomainname
@compute-1.internal relayer@hosteddomain.com
# output of hostname -f
@domU-01-02-03-04-05-06.compute-1.internal relayer@hosteddomain.com

/etc/aliases

# Run newaliases after changing
# Lot of stuff here. Mostly, just make sure the graph points to root, such as
mailer-daemon:  postmaster
postmaster:     root

# And the important part - your email or distribution group
root:           awsadmins@hosteddomain.com

/etc/passwd

# Sometimes it helps to expand the name, so email comes from 'root at aws host 5'
#  rather than just 'root'
# Was
#root:x:0:0:root:/root:/bin/bash
# Is
root:x:0:0:root on aws host 5:/root:/bin/bash

Things I'm happy about:

  • A lot of mail gets sent to root, and the one line in alias directs who gets it.
  • All mail from local users is translated to coming from relayer@hosteddomain.com, so it gets through the MS Online SMTP server.
  • postfix has much better documentation than sendmail.

Things I'm not happy about:

  • Custom changes are required for each host, and several steps. I wrote a bash script to help.
  • The passwd name trick doesn't always work, and it can be difficult to figure out what server a mail is coming from.
  • Every mail sent puts three warnings in the log:
    1. warning: smtp.mail.microsoftonline.com[65.55.171.153] offered null AUTH mechanism list (SMTP server sends a null AUTH list before STARTTLS, but AUTH LOGIN after).
    2. certificate verification failed for smtp.mail.microsoftonline.com: num=20:unable to get local issuer certificate (There are some config options around certs, but I'm not sure if mail delivery breaks when the cert is renewed)
    3. certificate verification failed for smtp.mail.microsoftonline.com: num=27:certificate not trusted (Same as #2)

Thanks to the serverfault community for sharing strong opinions on mail servers.

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I use canonical mapping to rewrite the from address, such as rewriting root@app01 to no-reply@example.com.

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3  
Welcome to Server Fault! We really do prefer that answers contain content not pointers to content. Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. –  Iain Dec 12 '12 at 14:43
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