Difference between “reply by email address” and “alternative reply by email addresses”
RBoy asked about the difference between the two settings in the Admin → Email page. supermathie explained that Discourse generates mail using the reply by email address setting, but accepts mail that matches that or any email in the alternative reply by email addresses setting.
RBoy clarified that by “accept” mail, it means processing email responses with the “to” address containing any of the emails from the alternative reply by email addresses setting. supermathie confirmed this and added that there is no need to duplicate the reply by email address in the alternative reply by email addresses setting.
Regarding the use of a reply_key, pfaffman questioned its necessity, and RBoy assumed it was optional. However, after testing, RBoy found that the reply_key is not necessary for email responses to be accepted, but without it, Discourse doesn’t use the email specified in the reply by email address setting. A bug was reported separately: Reply by email address broken.
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.