Suppose you have set up your Raspberry Pi to do some stand alone work. It would be nice if it could email you occasionally, for instance if there’s something wrong. Or it may send you status updates on the work it’s doing. For this you could use the SMTP server of your ISP. However, if your Pi isn’t stationary and roams around on multiple networks, this is not an ideal situation. If you connect your Pi to a different network, operated by a different ISP, chances are that you can’t send any messages.
A better solution would be to use a Gmail account for that. In this description I presume you already have a Gmail account. If not, you can get one for free at www.gmail.com. You may also decide to create a new one for your Raspberry Pi. The same account can even be shared among multiple Raspberry Pies. Rumour has it though that Gmail will only allow 100 mails sent per account per day. So if you share your account among too many machines you may start hitting the ceiling soon.

Full article here:
Prepare Your Pi To Send Mail Through Gmail (SB-Projects On Line)