First, congratulations for setting up your own server with SPF, DomainKeys, DKIM, and SenderID. Obviously you care about deliverability!
In this scenario, Postmark is not actually a viable option, simply because we deal specifically with transactional email (welcome emails, invoices and receipts, in-app notifications, etc.) where deliverability failure can lead to significant loss of sales, vs. "bulk" email like a marketing campaign where deliverability failure leads to potential opportunity cost, but not at the same level of severity.
It's possible that a sudden surge in email volume will trip the circuits on ISPs' radars, especially for a marketing campaign, so I would advise that you put some thought into the email itself to avoid an uptick in spam complaints that will damage your sending IP.
You can also consider throttling the campaign or sending it in stages rather than push it out as fast as possible, if that's an option for you.
Also, remember that SenderScore works like a credit rating; in many cases, sending low volumes of email is a lot like having "no credit history". A sudden spike may be over your threshold based on the email volumes you have sent in the past.
That said, the email safeguards you have already taken to verify your identity go a long way in the ISPs eyes, so keep that up, use best practices in your email design, send with some light throttling, and you should be okay.
ESPs (email service providers) like Newsberry or MailChimp are still good options for marketing campaigns if you want to use IP addressess that are specifically used for bulk email and actively monitored to keep their reputation high. This is the equivalent of sending through an IP that has already sent millions of similar emails, vs. one that is normally low volume. Plus you get click tracking statistics to improve your performance, etc.