>Hi. When working with "e-mail sending" programs, can you use anyone's smtp (ie: gmail, yahoo, hotmail)?

If you mean, can you specify anyone's SMTP server to handle your outgoing mail, the answer is almost always no, on the standard port 25. As part of an effort to reduce spam, most ISPs require that outgoing SMTP traffic go through their SMTP servers, which are usually so-called "smart hosts".

For example, if your site's Internet connection is provided by CompanyXYZ, they will usually require that outgoing SMTP mail on port 25 go through smtp.companyxyz.com or some similar, specified host on their domain. They will not relay outgoing SMTP traffic to other SMTP servers not in their domain.

Some SMTP servers accept connections on ports other than the standard port 25. However, to reduce spam they may require authetication i.e. a valid account name and password, and may require a secure connection (SSL) as well. To make use of these servers, your "e-mail sending program" must be able to meet the server's non-standard requirements.