Before Saints games, we send a preview email to the full DB. OR fluctuates depending on team performance (and clever copywriting), but has been fairly consistent over the last few seasons.
Before our first 2018 preseason game, OR dropped by 75% YoY. CLTR, on the other hand, was up 50%, with clicks up 150%. CTO was 70% -- a metaphysical impossibility in this universe, but maybe not in the multi-verse.
I submitted a support ticket since the OR was an absurdity. Other templates were humming along as usual. The game preview code didn't change much from last year, but I speculated that something in my clunky html/css may have screwed with the 1x1 tracking pixel, which I assume is appended by Marketo. We do use a bgr image -- a pattern -- and the emails are fairly image-heavy (example below), but nothing insane. I'm scratching my head. The OR, when combined with the CTO and CLTR, isn't possible.
I got a whole host of suggestions for improving deliverability (we're still on a shared send IP), including getting the NFL's domain authenticated with DKIM or SPF. "From the recipient side of things, if your team has been sending emails from Marketo at high volume for a period of time from a domain that does not have an authenticated DKIM or SPF record, it appears as though Marketo is not authorized to send emails on your behalf. This also increases the chance of your emails landing in the spam folder over time since the email was addressed from your domain, but sent from an IP address with a Marketo domain. Your team may have experienced strong Open rates in the past, but since receiving email clients can't validate that Marketo can send on your behalf, the receiving server will begin to place your emails in the spam folder as opposed to the inbox or promotion folder."