Complete Virtual Mail Server/Postfix additions

Introduction
At this point, Postfix should be running well and users through mailclients and webmail should have no problem sending and receiving e-mail securely. Tweaking the postfix installation can give a little more performance, make it a bit more secure and having a bit redundancy never hurts either.

Introduction to backup-mx
For redundancy postfix offers the feature of backup mx. Backup mx allows another mailhost to catch e-mail for a domain when the main mailserver is for whatever reason not responding. A backup mx server will try to deliver the mail to the original mailserver. This all sounds just perfect, however backup mx is dead. Spam killed it. The problem with backup mx nowadays is, due to spam, mail can end up bouncing the wrong way and the backup mx server may thus end up on a spam block list. This kind of mail is called Backscatter mail. If a secondary mailserver is setup, it needs to know all of the users from the primary domain so that it can bounce unknown users. A simple way to do this, is having the entire database synchronized between primary and secondary mailserver. Caveat being however the 'backupmx' flag in the domain table. This would need to be inverted on the secondary mailserver.

Configuring backup-mx
The database already knows about backup domains. If a domain has a backup mail server, the backupmx field is set to 1 in the domain table. Connecting postfix to the database requires a file with connection information.

This then needs to be configured into postfix.

To make proper use of relay_domains

Also this needs to be configured into postfix.

After restarting postfix, the mail server is now a less-open relay, relaying mail to only approved domains and approved users.

Introduction
uotas are a tool to help a user monitor their mailbox. They are very easily abused and should not be relied on. That said, they can work well enough most of the time and give the user feedback on the status of his usage pattern. IMAP supports quota reporting and thus the mail-client can even report this the user. Thunderbird does this via an extension, roundcube shows this per default.

Configuration
he database supports mail quotas however postfix requires a patch to support these mailquotas. The vda USEFlag enables this patch for postfix and allows the use of quotas.

First, the data needs to be obtained from the database.

There are a few things that need to be configured in postfix, next to the database query. The Trash folder for example, should not be counted when automatically deleting Trash after a fixed amount of time. This can (and has) be done in courier-imap.

Testing
Roundcube displays diskusage per default and hovering over it displays detailed information. Thunderbird has an extension, Display quota. For both and others to actually work a maildirsize file is required. This file will be created and updated whenever postfix delivers a message or when courier-imap makes chances. The file is located in each virtual users root mail dir, which would be in the case of testuser on example.com. Thus sending a message to testuser@example.com would create this file in the users maildir.

HELO Restrictions
Hackers, spammers and everybody else can obtain information from the mailserver by using the HELO and EHLO commands. Spammers usually put fake information in the HELO greeting and thus restricting and rejecting connections from servers that do not properly identify themselves can only be good. Postfix offers the smtpd_helo_restrictions variable to tune how to respond to connections. Restrictions here have to be thought over carefully however. Locking down the server too tight may make it so messages are rejected because the other server is just poorly configured. Use with regression. A quick overview of the available options.

 smtpd_helo_restrictions overview
 * reject_invalid_hostname	 REJECT if HELO greeting is not a DN or an IP
 * reject_non_fqdn_hostname REJECT if HELO greeting is not a FQDN or an IP
 * reject_unknown_hostname	 REJECT if HELO host has no A or MX in DNS
 * permit_naked_ip_address	 PERMIT if HELO greeting is an IP

The following table should give an idea how mail will get rejected from the various restrictions. An X mens the message will be rejected, the ? means it depends on the proper DNS record setup for that domain and O means the message will be delivered normally.

On a properly configured network the following will be tight and should work.

Deny local username farming
Normally postfix, or general MTA's allow to verify whether a mailbox exists or not. This command may have been useful in the early days of mail, but is almost exclusively used by peole who maintain bulk mailing lists and search if accounts are still valid. This command can be disabled by postfix.

After a restart of postfix, telnet to port 25 will no longer show 250-VRFY.

SMTPD Banner
Another oftenly abused feature is the SMTP header. Also some countries require senders to honor the NO UCE greeting message (No Unsolicited Commercial E-mail). Also it is wise not to tell any outsiders what MTA is being used or what version thereof. Postfix allows for changing the SMTP header quite easily. Find the smtpd_banner section in main.cf.

Message size
For years the default message size amongst MTA's has been 10MiB. Google raised the bar with their gmail service to 20MiB per message. If bandwidth isn't an issue this can easily be accomplished with postfix.

Biff
For compatibility reasons postfix's local mail notification is enabled by default. With many users this can be a performance drain and since there are no local users to use the biff command anyway this can safely be disabled.

Processes
The amount of concurrent processes of any of postfix's applications is limited to 50. The first bump in high load environments can be very quickly be the amount of active daemons. Smaller setups should not need to worry about this setting.

DNS Lookup
A common throughput limiter is the use of DNS lookups. These can be slow and can be an issue far before processes, CPU or memory are the issue. One lookup per message is required at the very least per message, a server MX record needs to be found. A local caching DNS server could help enormously here and packages such as or even a full  setup should be used.

Mail Storage
Postfix maintains a number of queue directories in /var/spool/postfix. Various postfix applications pass messages around each other using these queues. Placing these directories on a separate disk, raid array or an SSD can significantly improve overall throughput. Also making sure that the partition in question is marked with the noatime option helps a lot on disk access. Postifx does not use access timestamps.

Multiple mail servers
If one server is not enough, multiple mail-servers can spread the load. Also having multiple mail-servers offers redundancy so it is something worth considering. spreading the load over several mail-servers can be done effectively using DNS round-robining. Two options are available here, either assign multiple A entries to the mail-server, or cleaner, have multiple MX entries which allow prioritization.