Problems with forwarding mail
It is easy to set up mail redirections to forward mail onto an existing email address. For example, you may choose to forward email@example.com to firstname.lastname@example.org where mydomain.com is hosted on our servers, but otherdomain.com is not. When this is in the place the following happens when email@example.com sends an email to firstname.lastname@example.org:
- Mail server for senderdomain.com (assume called mail.senderdomain.com) looks up the mail server designated to handle mail for mydomain.com. This will be one of our servers.
- mail.senderdomain.com sends mail to our mail servers
- Our mail servers look up the mail server for otherdomain.com (assume called mail.otherdomain.com).
- Our mail servers send mail to mail.otherdomain.com addressed to email@example.com
Compare this to when firstname.lastname@example.org sends an email to email@example.com directly:
- Mail server for senderdomain.com (assume called mail.senderdomain.com) looks up the mail server designated to handle mail for otherdomain.com (assume called mail.otherdomain.com.
- mail.senderdomain.com sends mail to mail.otherdomain.com addressed to firstname.lastname@example.org
Sender Policy Framework (SPF) allows you to tell people which mail servers will send out email from your domain. This is entirely optional, but it allows the recipient servers to spot if someone has spoofed up your email address (e.g. a spammer) as they would not be sending from your 'official' mail servers. Problems arise if the final mail server (e.g. mail.otherdomain.com) blocks email that does not come from your 'official' mail servers.
In the above example, let us assume that the person in charge of senderdomain.com declares that senderdomain.com mail will only come from mail.senderdomain.com. You will see that when delivered directly, this is fine, but when forwarded the mail will come from our servers not mail.senderdomain.com. Therefore, the mail might be rejected. Google, in particular, is very strict about this and so forwarding onto a gmail.com or googlemail.com address is unlikely to be successful.
Note that the SPF record is under the control of senderdomain.com and the checking is done by mail.otherdomain.com. Neither of these are anything to do with you, mydomain.com or our servers and thus there is nothing you can do to prevent this (besides not use mail forwarding).