My friend and I heard someone on the phone say the phrase 'mailbox money'.
The sentence was 'mailbox money is always nice'
My friend told me that 'mailbox money' means money/checks that you keep getting in the mailbox (not necessarily in the mailbox, but usually), by doing only one thing once. For example, if you developed an app, you would work on it once, but keep getting money/checks for it. Same goes for developing a website and putting some ads on it. (I understand that websites need maintenance, and apps need to be updated, but the general idea is that you do something once and you keep getting money/checks for it.)
When I Googled it I could not find a really good website/reference for any legit definition for that phrase. I read on this website that it means money that you're not expecting.
Any enlightenment?
Best Answer
It's not exactly a well-established expression, but the meaning is reasonably transparent.
Here's an online article from 2005 which I think makes it clear "mailbox" can be seen as metaphorical. Essentially, it just means income (often sporadic, relatively small amounts from many different people) that keeps arriving long after you did something.
It's very much a "how to make money on the Internet" sort of term. I don't think a musician, say, who periodically receives royalty money from something he recorded decades ago, would call it "mailbox money".
A plausible context would be where you create a website that becomes popular, and sell advertising space on it. Assuming people keep going to your website, you should keep getting payments from the advertisers. But as you can imagine, in practice it's probably not as simple or as lucrative as it might seem. Most likely the only person getting rich is the guy who persuaded you to send him $10 for his book explaining how to create a popular website.
TL;DR: "Mailbox money" is an Internet marketing myth.