What is the difference between “reply by email address” and “alternative reply by email addresses” in the Admin → Email settings page?
Also, if I was using a reply_key earlier in the reply-to email, but now I’ve switched a reply-to email without a reply_key, do I still need to enter the older reply-to email address with the reply_key in either of these two fields or I don’t need the older email address anymore? (I’ve unselected the “find related post with key” option with the new email address)
Discourse will generate mail using the reply by email address setting, but will accept mail that matches that or anything in the alternative reply by email addresses setting.
(I’m not sure of the answer to the second question)
Thanks, for clarification; when you say accept mail, do you mean that it process an email response with the “to” address containing any of the emails from the alternative reply by email addresses setting?
Also, does on need to “duplicate” the *reply by email address" in “alternative reply by email addresses”? I ask because the example provided on the settings page does seem to replicate it.
Correct 
No need to duplicate them.
Isn’t the reply key necessary for the posts to be connected to the right topic?
Good question - I’m assuming it is optional which is why there another option on the Admin Email settings page - Find related post with key
I thought it uses a combination of the topic title and email address to match it up when it’s not using the key.
So the short answer is yes, it does work. You don’t need the reply_key to get it to accept email responses to messages. However I did find a bug as without the reply_key it doesn’t use the email specified in the reply by email address. I’ve reported that separately here.
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