There is no detailed plan for leaving the EU at the moment, one reason being that there cannot be until the negotiations post implementation of Article 50 come to at least some conclusions. However, for email marketing it could be a less bumpy ride than for many other businesses.
One thing must be accepted; it is very unlikely that everything will be settled in two years. The process will continue for some time, with staged changes allowing systems to adapt smoothly. There is the possibility that the EU will be a bit stroppy in negotiations, in fact almost a certainty, but they don’t want to have disruptions to one of their biggest markets.
Data protection should remain unchanged. Whatever happens, we will trade online with the EU and so will be obliged to conform to their requirements. The GDPR is a shoe-in. So keep working on the implementation plans.
The initial stage of the leaving process will be for us to pass the legal framework of the EU into the UK law. This is something of a must in order to lower disruption. Changes can be made at a later date and with consideration.
In other words, it is probable that initially there will be no altrations and this will be embedded in law. So nothing for us to do initially. Also, one would expect the markets to reflect the move.
Next will come the type of relationship we will have with the EU. The Norwegian Option has been mentioned, as has the Swiss/Turkey route, but there are alternatives, such as EFTA. In fact quite a few. One worrying point is that the EU will soon be renegotiating its treaties. What that means is unclear.
There is a sort of long-stop, trading with the EU under WTO rules. This requires that we gain no advantage and comply with their regulations.
The relative likelihood of each of these options being the chosen one is unknown.
We need a smooth transition, keeping regulations and laws as near to how they are currently. This will reassure the markets and allow trade to continue without undue disruption or additional expense.