Subdomains

Addressing with subdomains

When you use a your own domain, you can control how FastMail.FM handles sub-domain addressing for your domain. The last column on the domains screen labelled Subdomain? a@b.dom as: controls this. The options are:

  1. Yes, swap

    Sub-domain addresses are handled like regular FastMail.FM domains, that is:

    blah@something.yourdomain.com -> something+blah@yourdomain.com

  2. Yes, drop

    Sub-domain addresses are treated as if they were regular domain addresses, that is:

    blah@something.yourdomain.com -> blah@yourdomain.com

  3. No, reject

    Sub-domain addressing is disabled for your domain.

When sub-domain addressing is disabled for your domain, this can mean two things:

  • If the DNS for a sub-domain points to us, then sending to that sub-domain will be rejected and bounced.
  • If the DNS for a sub-domain points to other servers, then sending to that sub-domain will direct email to the appropriate servers.

Handle different subdomains separately

It is possible to set only some subdomains to be handled by external servers. As an example, say that you have yourdomain.com, but have set the DNS for lists.yourdomain.com and forum.yourdomain.com to be handled by different mail servers (eg MX records point to other servers). In this case you have two choices:

  • If you want xyz@yourdomain.com to be handled by us, lists.yourdomain.com and forum.yourdomain.com to be handled by external servers, and email to @anythingelse.yourdomain.com to be rejected:

    1. Set the MX records for yourdomain.com to point to our servers ;
    2. Set the MX records for lists.yourdomain.com and forum.yourdomain.com to point to the external servers;
    3. Set the SubDomain handling for your virtual domain yourdomain.com to be "No".

  • If you want xyz@yourdomain.com to be handled by us, lists.yourdomain.com and forum.yourdomain.com to be handled by external servers, and email to any other sub-domain (eg @anythingelse.yourdomain.com) to be handled by us as a normal sub-domain, you need to:

    1. Set the MX records for yourdomain.com to point to our servers ;
    2. Set the wildcard MX records for *.yourdomain.com to point to our servers ;
    3. Set the MX records for lists.yourdomain.com and forum.yourdomain.com to point to the external servers;
    4. Add the domain yourdomain.com as a virtual domain with Subdomain handling set to "Yes, permute";
    5. Add the domains lists.yourdomain.com and forum.yourdomain.com as virtual domains, both with Active set to "None".

    This will tell FastMail.FM to handle sub-domains in general for your domain, but explicitly exclude lists.yourdomain.com and forum.yourdomain.com and have them treated as external domains.