"relay as" domain rewrite in new smtpd.conf syntax

[Allan Streib]

Prior to 6.4, in smtpd.conf(5), the relay directive supported the "as"
parameter:

    If the as parameter is specified, smtpd(8) will rewrite the sender
    advertised in the SMTP session. address may be a user, a domain
    prefixed with ‘@’, or an email address, causing smtpd(8) to rewrite
    the user-part, the domain-part, or the entire address, respectively.

In the new smtpd.conf(5) syntax, how is that rewrite achieved,
specifically the "@" prefix behavior to rewrite the domain part?

Thanks,

Allan