Is it bad to only sign some of a domain's emails with DKIM?

How do I create a DKIM signature for email in Media Temple?

• I send emails from two sources: 1. Gmail (three work emails so Google Apps supports sending emails from those three work email addresses with my domain) and 2. Emails generated by my site like invitations, responses to comments, weekly updates, etc.  I have a SPF Record for both, but I only have a DKIM signature for the three emails from Google Apps. I'm trying to figure out how to create a DKIM signature for emails geenrated by my domain on Media Temple like from an email from . Again, I have a SPF Record, just no DKIM signature.  Any help will be appreciated. Thanks

• Answer:

  SPF is much easier since you only need to know the source IPs that your email is coming from. DKIM, on the other hand, is harder since each message must be signed as it is prepared and sent. You would have to set this up in Postfix. Another option is to use an email service such as Postmark, Mailgun or Mandrill. We (Postmark) focus on transactional email only and our process is very much catered toward authentication such as SPF and DKIM. Feel free to ask any follow up questions. Here is a link to Postmark: http://postmarkapp.com