Many online sites such as news-sites and other content providers often have a “tip a friend” option. With this you can mail a friend and tell them about an interesting piece of content you’ve found. The Idea seems quite simple, and everyone should have the tip-option, wright? - no, wrong. While it may offer a convenience for some, it has several backsides.
First if you - or your email provider - has implemented anti-spam techniques such as SPF-records, the “tipping mail” will not be sent through the authorized list of mail-servers and thus have a larger likelihood of being labeled as spam. Your mail be sent, but you don’t know if it will arrive in your friends mailbox.
Second having two (assumable) valid email addresses submitted to a site could be a goldmine for evil spammers. Besides mailing the tip, the email addresses may be collected and abused some time in the future.
Third by having a site sending thousands and thousands of “tips” to friends does mimic a spammers’ behavior - the function may cause your servers to be labeled as a spamming server, and you may not be able to send mails - no tips nor important messages your servers may try to mail to you or other users.
If you want to have a “Tip a friend” function - make sure it’s a mailto-link.
The mailto-link may not be as sexy as the options available when you create the mails server side, but the links go out from the users’ own mail client and the likelihood of it being labeled as spam is far less. It also gives the user complete control of what is being sent - no unexpected ads or anything else unwanted material.