Aliases mail domain and the form forgotten password

I wouldn't do it. Here is an example: there is such email provider is fastmail.fm (recently bought Opera). He has a lot of domains (fastmail.fm realemail.com, mailc.com etc.). To some point all these domains were aliases. That is, emails sent to xxxxxx@fastmail.fm, xxxxxx@realemail.com, xxxxxx@mailc.net got the result in the same drawer. But one prekrasny the time (sometime in 2005) Fastmail decided to "share" a domain with all its consequences. It was a lot of confusion. You can't predpolagat that if 2 domain aliases today, they will remain the aliases tomorrow.
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There is one caveat: on some standard emails sent to xxxxxx@domain.com and xxxxxx+yyy@domain.com fall in certain cases in the same mailbox. This trick works with gmail but not works with most other email providers (which do not support this strange RFC). What will you do in this case? After all, it's not even clear which providers support this.
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P. S. Add to your list of aliases: aol.com and aim.com aliases from the point of view email (email sent to xxxxxx@aol.com and xxxxxx@aom.com falls into the same mailbox).

Here's another: after AOL killed Netscape email service from the owners email accounts @netscape.net said that for each xxxxxx@netscape.net created xxxxxx@aol.com (concurrent registration xxxxxx@aol.com and xxxxxx@netscape.net was impossible because @aol.com was in due time paid, versiya @netscape.net) and email sent xxxxxx@netscape.net come on xxxxxx@aol.com. Immediately after the dissolution @netscape.net, AOL made email service @aol.com / @aim.com free. But for those users who have never had a box on @netscape.net alias does not work. Also alias @netscape.net not working for new users @aol.com/@aim.com. To have even within the same email domain-you can't define an alias or not.