The only issue with this is Google's sending limit (which I believe is ~500 messages per day, perhaps it's even less through SMTP). That could be bad if you wish send an opt-in email newsletter, for example. Also, Google passes the X-originating-IP header on all mail sent through its SMTP servers (at least it did the last time I tried this), which can easily be a one way ticket to the spam box for all of your outgoing emails.