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I own these domains. They are all on my servers -- do I have to buy a new version of sendy for each one? I dont want to send out florida emails to atlantic etc.
And I am not sure how amazon works if I need a separate account for each domain
Sendy's licensed domain is just where you install Sendy on, that's all. You can send from any email addresses you like on a single Sendy installation.
If you meant, you want to install Sendy on each of these domains - florida.com, atlanticcity.com, whowhatwhere.com and artsandcrafts.com then yes you need to purchase a license for each domain to install Sendy on each domain.
I didnt know I can send as an "authorized" sender from another domain other than installed domain
It's written in the FAQ on http://sendy.co