Switching email vendors is usually no small task. There are a lot of things to remember; email templates, images, segmentation, forms, etc. all need to be moved. Users will need to get acquainted with the new system and interface. You will also want to think about compliance, and what you need to consider may depend on where you recipients are located.
One tricky part of the transition to manage is subscription and unsubscribe methods. You will need to have a plan for this and there will need to be some overlap between the end of the old vendor and the start of the new vendor. You will want to import unsubscribes and spam complaints into the new vendor so that suppressions are accurate. Most likely there will be some data syncing for unsubscribes and subscriptions between both vendors during the transition. There are many ways to handle this transition but usually some planning is required.
For example, what about new unsubscribes from the old vendor? You are already sending out of the new vendor, but those emails you sent last month or last week from the old vendor may still be viewed by recipients and they may unsubscribe. You need a way to honor those unsubscribes.
In the US, CAN-SPAM requires the unsubscribe link to remain active for 30 days. In Canada, CASL requires the unsubscribe link to remain active for 60 days. In the EU, GDPR does not provide a time limit for the unsubscribe mechanism but states that withdrawing consent must be as easy as giving consent.
You will want to pick a timeframe you are comfortable with and one that meets your compliance requirements (30, 60, 90 days, etc.), where you collect unsubscribes from the old vendor and sync them to the new vendor. I recommend doing this at minimum on a weekly basis but more frequent than that would be better. What you don’t want to do is switch from the old vendor to the new vendor and then immediately switch off your access to your data at the old vendor. I would also recommend either moving over or exporting other data from the old system before you lose access. Engagement data – opens and clicks (possibly delivers, if you have it) – will be key for segmentation going forward. You don’t want to give up that data and start from scratch just because your forgot to export it.