Use authenticated SMTP / SMTP-AUTH

Normal SMTP (outgoing mail) servers can only be used while you are using your ISP's Internet connection - this is normally done as an anti-spam precaution.

Unfortunately this causes problems if you use multiple ISP's (i.e. mobile users or if you use one ISP at work and another at home). This means you would have to change your SMTP settings to match the ISP you are using.

To get around this you can use an authenticated SMTP mail relay - like AuthSMTP. This authenticates you using a user name / password and allow access using alternate SMTP ports - meaning you can use it from almost all ISP's (i.e. when your ISP blocks the default TCP/IP port 25 used by SMTP).

Note: Authenticated SMTP is also known as SMTP-AUTH - for more details see here.