If I send an email from one of my aliases (using FM's webmail system) nothing in that email tells the recipient about my FM account email address.
If someone else sends an email to my alias, then in the email I receive there are headers which show it was sent to the alias, but placed in the differently-named mailbox for my account. For example such an incoming email would contain
X-Resolved-to: myaccountuserid@oneofthefmdomains
X-Delivered-to: thatalias@whicheverfmdomainitis
If you're trying to keep the underlying account userid a secret (which I certainly do, NEVER using it for email) then it would be stupid to forward such an incoming email anywhere else.
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