Would it be possible to user an email header value in a content filter at the domain level to curb spoofed emails coming into and going out of that domain?

e.g. place a header with a value like "valid" in all emails for "exampledomain.com" and then set a content filter to allow emails from "exampledomain.com" with header "valid" and to send all other emails from "exampledomain.com" to a junk folder?

Actually if "exampledomain.com" has an SPF record, SmarterMail will take care of this automatically for you.

Other than that, there's not too much you can do ahead of time via content filtering to determine if a message is "valid" or not - at least, not simply based on what is in the headers.

We use a two-step pair of rules for a situation similar to this problem.

The FIRST RULE of the pair, is our list of exempt e-mail addresses. In the Edit Content Filter window, the Filter Type is set to "OR", no wildcards, then Criteria Details - From Address is set to "matches" ... and then the list in the "From addresses" field, is of our company members' e-mail addresses. The action for this FIRST RULE, is to move the captured e-mails to the Inbox.

The SECOND RULE of the pair, is a list of our company domain names. In the Edit Content Filter window, the Filter Type is set to "OR", we use wildcards, then Criteria Details - Contains Specific Words or Phrases is set to "contains" ... and then the list in the "From Address" field, is our company domain names. For example:

*@abcexample.com
*@abdexample.com
*@xyzexample.com

The action for this SECOND RULE, is to move the captured e-mails to the Junk E-mail folder (where they are automatically marked as Spam).

In this way, if somebody from our company, sends an e-mail to somebody else at our company, it is considered "exempt" (from a "known good one") and routed to the recipient's Inbox; but, if some bogus-so-and-so@xyzexample.com is who the e-mail is allegedly from, it is routed to the Junk E-mail folder.