You cannot use Gmail or Yahoo mail email servers as these Email servers only accept SSL connections. This does not mean you have to change your Email accounts just to send Emails through our program. You only need to use a different outgoing Email server while still keeping your Gmail or Yahoo Mail account.
INTERNET SERVICE PROVIDER (ISP)
Most Internet Service Providers do provide an SMTP our Outgoing Email server that you can use. You need to check with them for their outgoing Email server settings.
3TD PARTY EMAIL SERVICE PROVIDERS
If you cannot get it to work with your Internet Service Provider, you can also try some 3rd party service providers like SMTP2GO.com. You can click here and scroll to the bottom of the page. At the time of this writing they provide a free service of up to 20 emails a day.
If you are currently able to send emails using Outlook or Windows Mail, you can set Instant Invoice to use MAPI and Instant Invoice will relay the outgoing emails through these programs.