IT support friend of mine told me a story how the place where he worked (major US bank) basically took its whole email infrastructure offline when the company essentially DDOSed itself with a single email.

Someone in IT was testing an email distribution list for the whole company (CEO announcements, that sort of thing) and sent out a test email to the effect of “Hello, did this work?”.

Two problems.

1. They had emailed the whole company for real, not the test list they thought they emailed

2. The list was setup such that anyone in the company could email it and also send a message to all other employees in the bank.

Within about 5 minutes everyone in the bank had hundreds of emails turning up. Initially this was people saying “Yes I got it”. Before long people were emailing to say “Please stop sending all these emails” which in turn got sent to the whole company.

Of course some people then started replying all to tell people to stop replying which only added fuel to the fire. The exponential traffic growth effectively brought the email system to halt for many hours until the mess could be cleaned up.