If your ISP requires SMTP authentication for sending mail, check this option in your e-mail client. Check this option if applicable. Other settings Other settings are at your preference. Explore the options or preferences panel of your e-mail client, not just initially, but as you get more comfortable. If you can't switch for whatever reason, look at the rest of the list.
Most problems come from incorrect entries for incoming or outgoing mail servers.
Don't rely on the fact that things have been working until now. Sometimes ISPs change the server name or prefix and forget to inform you. In other words, if you have two ISPs, or are visiting a relative, you need to switch the SMTP server setting to the ISP you are using. The problem might also be various authentication settings. Again, check with tech support Not all e-mail clients support all authentication methods.