I have a Google Mail account that I have set to automatically forward to my Yahoo Mail account. I also have a different Yahoo Mail account that forwards to a different GMail account. It got me thinking. If I were to have Yahoo Mail automatically forward to a GMail account that automatically forwards back to the same Yahoo account, what would happen? Will both accounts fill up until there’s no more space? Will the internet explode?
The path gets recorded smtp mail header, and modern MTAs (Mail Transport Agent) generally implement some form of loop detection. Your email will get stopped.
Otherwise, your email boxes fill up with copies of the email. This will stop the process. The internet will not explode. 
Si
Thank goodness. I’d hate for terrorists to be able to bring us down by sending one fateful email.
It’s known as mailbombing yourself. All mail servers put in protection against mailbombing at least a decade ago.
I remember the good old days when “out of office” loops occasionally happened.
I’d send my customers an email telling them that I was about to go on vacation and that you should call Bob if you run into any problems, then I’d set up my email account to generate an automatic “I’m out of the office until Monday” response to all incoming email.
Unfortunately, one of my customers would be on vacation and had already set up his email account to generate a similar automatic “I’m out of the office until Monday” response.
…and the loop would begin.
(“Out of Office” responses are supposed to include a flag in their email headers that essentially tell the recipients “don’t autoreply to this message”. In the good old days not every mail server put in those flags and not every mail server paid attention to them.)
As an aside, you can get a great free tool called PopPeeper. It works with webmail like Yahoo and Pop/IMap like gmail.
You may not need to forward with this small free tool
I did this to reply to an idiot friend of mine, who had turned on some moronic “out of office” reply. Every time I got an email from him, with his out of office message, I would send an email back saying “Stupid out of office message.” Of course, this would generate an automatic reply from his end.
Lather, rinse, repeat.
I was disappointed that this system stopped my hilarity after around 10 cycles…