You need to know their password and then just log in as them. I hope by "email marketing" you do not mean SPAM.

That's a fairly vague question, unfortunately. When you say that you want to use someone elses' email address, are you saying that you want the email to appear to be from them, when it is in fact from your account?

I'm wracking my brain trying to figure out a single legitimate use for something like that, and I really can't think of any…But, I'm sure you've got your reasons, and I won't let my morals stand in the way of enlightening you…

The simple answer to your question is that there is no way to do what you're describing using a simple email program like Outlook or a web-based program like Yahoo! Mail. You would need to have both webspace, (i.e., your own website, with server space), as well as a PHP script that will allow you to send emails. That obviously means that whatever webspace you have must allow you to use PHP scripts.

If you're still interested in walking down a complicated path, and wouldn't mind giving me a more detailed idea of what you'd like to accomplish, IM me, I may be able to help.

I **** to break it to you, But they (spammers) are already doing that they send the message to a bad address with the target as the return address. The message arrives in the targets mailbox saying undeliverable message and the user most of the time opens the spam message. If you get caught by your ISP most likely they will end your account and black list you.

To do this you need to know a bit of programming, and be familiar with SMTP (simple mail transfer protocol). A good way to determine the format for the message, it to locate and familiarize yourself with the RFC for SMTP.

The RFC specifies the format, structure, and process for sending email. Once you've programmed the file structure then you need to submit the file to a SMTP server. But since most servers are password protected these days, to prevent such a thing, you will have to own or find a suitable target.

Your best bet is to establish a free email account at a hosted site and spam away until they shut you down.