The thing is Reverend is not a title, it is not used like 'Dr' for example. It is in fact used in pretty much the same way as 'Honourable' for Members of Parliament. So next time you use it in conversation do the MP check in your head. I haven't checked how to email an MP, but let's get to letters.
A letter to a Vicar would be addressed to:
The Rev'd Lucy BruceAnd begin
Dear Ms Bruce,However an email has no way of indicating the orders of the clergy-person being written to. Vicars themselves should have no problem with this, but some people writing to Vicars obviously do. Otherwise they would not have asked the question on Twitter.
Personally I am happy with Dear Father (Or Mother) but this does not sit well with all traditions. I suppose Dear Parson might just work if you're rural.
From the wider church I can see two options. Firstly we have the example of Deans who you write to as
Dear Ms Dean,So that would begin your email with
Dear Ms Vicar/Rector/PriestInChargeTillNextTuesdayI am not convinced, so I think we follow the way we all write to Archdeacons (carefully).
Venerable Madam,So that's my advice. If you can't bring yourself to write to your local Vicar by email as
Dear Mother Lucy,Then email her with