It's a shorthand appropriate to some cases.
Certainly, the meaning of @ has changed. When it was used in email addresses, Ray Tomlinson was punning: the meaning of @ was "each at" referring to prices only, but it was called at and so e.g. my email address being jon-at-hackcraft.net* suggests that I am "at" that host.
When email became ubiquitous other uses of it to replace "at" in other contexts started to develop. Nightclub posters might use it to say where or when an event is at, for example. These were originally being deliberately non-conventional, and then frequent use made it merely informal.
Twitter and Stack Exchange similarly use it to address a remark "at" someone.
From this, it would be understood by many in an email exchange (particularly if they were familiar with Twitter), so you could use it if the tone was informal, and the audience tech-savvy.
It certainly isn't standard English of the sort that would be appropriate in all contexts. As long as you know its a shorthand only appropriate for some informal contexts, especially those where heavy abbreviation would be considered acceptable, then there is no harm in using it.
*Though the dot-separated hierarchy of hosts was a later thing still, so that's a proleptic example.