Many email clients (Outlook, Thunderbird, mobile clients) use the Autodiscover protocol to automatically configure mail server settings. While Microsoft Exchange popularized this feature, the underlying XML-based mechanism can be replicated for non-Exchange setups.

To implement this on Linux:

• Web server (Apache/Nginx) with SSL
• Valid SSL certificate (Let's Encrypt recommended)
• Access to DNS records for your domain
• Basic understanding of XML structure

First, set up these DNS records:

autodiscover.example.com.  IN  A      192.0.2.1
_autodiscover._tcp.example.com. IN SRV 0 0 443 autodiscover.example.com.

Here's a basic template for IMAP/POP3 servers:

<?xml version="1.0" encoding="utf-8"?>
<Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/responseschema/2006">
  <Response xmlns="http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a">
    <Account>
      <AccountType>email</AccountType>
      <Action>settings</Action>
      <Protocol>
        <Type>IMAP</Type>
        <Server>mail.example.com</Server>
        <Port>993</Port>
        <SSL>on</SSL>
        <AuthRequired>on</AuthRequired>
      </Protocol>
      <Protocol>
        <Type>SMTP</Type>
        <Server>mail.example.com</Server>
        <Port>587</Port>
        <SSL>on</SSL>
        <AuthRequired>on</AuthRequired>
        <UsePOPAuth>on</UsePOPAuth>
        <SPA>off</SPA>
      </Protocol>
    </Account>
  </Response>
</Autodiscover>

For Apache:

<VirtualHost *:443>
    ServerName autodiscover.example.com
    DocumentRoot /var/www/autodiscover
    SSLEngine on
    SSLCertificateFile /path/to/cert.pem
    SSLCertificateKeyFile /path/to/privkey.pem
    SSLCertificateChainFile /path/to/chain.pem
    
    <Location /autodiscover/autodiscover.xml>
        Header set Content-Type "application/xml"
    </Location>
</VirtualHost>

Use these tools to verify:

1. Test URL: https://autodiscover.example.com/autodiscover/autodiscover.xml
2. Microsoft Remote Connectivity Analyzer
3. Outlook test account creation

For dynamic responses, create a PHP script:

<?php
header('Content-Type: application/xml');
$email = $_GET['email'] ?? '';
?>
<?xml version="1.0" encoding="utf-8"?>
<Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/responseschema/2006">
  <Response>
    <!-- Dynamic content based on $email -->
  </Response>
</Autodiscover>

While Microsoft Exchange popularized the Autodiscover protocol, the underlying XML-based service discovery mechanism can be implemented independently for any email system. The protocol follows a simple principle: clients request autodiscover.xml from predetermined URLs to automatically configure email accounts.

Here's a minimal working example for an IMAP server:

<Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/responseschema/2006">
  <Response xmlns="http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a">
    <Account>
      <AccountType>email</AccountType>
      <Action>settings</Action>
      <Protocol>
        <Type>IMAP</Type>
        <Server>mail.yourdomain.com</Server>
        <Port>993</Port>
        <LoginName>%EMAILADDRESS%</LoginName>
        <DomainRequired>off</DomainRequired>
        <SPA>off</SPA>
        <SSL>on</SSL>
        <AuthRequired>on</AuthRequired>
      </Protocol>
      <Protocol>
        <Type>SMTP</Type>
        <Server>mail.yourdomain.com</Server>
        <Port>587</Port>
        <LoginName>%EMAILADDRESS%</LoginName>
        <DomainRequired>off</DomainRequired>
        <SPA>off</SPA>
        <SSL>on</SSL>
        <AuthRequired>on</AuthRequired>
        <UsePOPAuth>off</UsePOPAuth>
        <SMTPLast>off</SMTPLast>
      </Protocol>
    </Account>
  </Response>
</Autodiscover>

For Apache, create a .htaccess file with:

<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteCond %{REQUEST_URI} ^/autodiscover/autodiscover\.xml$ [NC]
  RewriteRule ^(.*)$ /path/to/autodiscover.xml [L]
</IfModule>

For Nginx, add to your server configuration:

location = /autodiscover/autodiscover.xml {
    alias /path/to/autodiscover.xml;
    default_type application/xml;
}

Add these DNS records to improve client discovery:

autodiscover.yourdomain.com.  IN  CNAME  yourdomain.com.
_autodiscover._tcp.yourdomain.com.  IN  SRV  0 0 443 yourdomain.com.

Use curl to verify your setup:

curl -v https://yourdomain.com/autodiscover/autodiscover.xml

Or test with an actual email client using these common endpoints:

1. https://yourdomain.com/autodiscover/autodiscover.xml
2. https://autodiscover.yourdomain.com/autodiscover/autodiscover.xml
3. http://autodiscover.yourdomain.com/autodiscover/autodiscover.xml

For a multi-tenant setup, create a dynamic PHP script:

<?php
header('Content-Type: application/xml');
$domain = $_SERVER['HTTP_HOST'];
?>
<?xml version="1.0" encoding="utf-8" ?>
<Autodiscover xmlns="http://schemas.microsoft.com/exchange/autodiscover/responseschema/2006">
  <Response xmlns="http://schemas.microsoft.com/exchange/autodiscover/outlook/responseschema/2006a">
    <Account>
      <AccountType>email</AccountType>
      <Action>settings</Action>
      <Protocol>
        <Type>IMAP</Type>
        <Server>mail.<?php echo $domain; ?></Server>
        <Port>993</Port>
        <LoginName>%EMAILADDRESS%</LoginName>
        <DomainRequired>off</DomainRequired>
        <SPA>off</SPA>
        <SSL>on</SSL>
        <AuthRequired>on</AuthRequired>
      </Protocol>
    </Account>
  </Response>
</Autodiscover>