Mail aliases expand as abbrevs---that is to say, as soon as you type a word-separator character after an alias (@pxref{Abbrevs}).  This expansion takes place only within the @samp{To}, @samp{From}, @samp{CC}, @samp{BCC}, and <del>@samp{Reply-to}</del> <ins>@samp{Reply-To}</ins> header fields (plus their @samp{Resent-} variants); it does not take place in other header fields, such as @samp{Subject}.