I am currently gathering requirements for a small (hopefully) project of migrating from SendGrid to Mandrill as a transactional email service provider. We have been using SendGrid for close to 3-4 years now and average about 5k-10k emails per day. We have SPF and DKIM records configured appropriately and, as a result, have very low bounce / spam rate and quite a good reputation as a sender.
There has been a decision to move to Mandrill and now I must ensure the migration happens smoothly with as few service interruptions as possible, thus starting the approval / reputation process all over again.
I know for SPF entries, it's possible to add multiple items, so, for the time being I'll keep both for SendGrid and Mandrill. However, I'm not 100% sure about DKIM entries. Some services recommend CNAME entries while others suggest TXT entries.
This leads me to wonder if it's possible to have a CNAME DKIM entry for one service and a TXT DKIM entry for the other. I'm curious about the effects of such a change. Is the vetting/verification of this entry wholly dependant on the intermediary / recipient? Or, do they generally see both and just pick the first one?
Essentially, what I'd like to do is find some way to slowly transition from one service to the next with as little interruption as possible. We have had issues before with ISP blacklisting and I'd very much like to avoid that.
Thank you so much for your time!