Gmail's Warning for Suspicious Email Addresses

A few days ago, I received a spam message in Gmail. Instead of flagging the message as spam, Gmail displayed this warning: "Be careful with this message. Someone might be trying to trick you by using similar-looking characters (such as Σ and E) in the email addresses contained in this message."

Last year, Gmail added support for non-Latin characters in email addresses. It also wanted to make sure that the new feature wasn't "abused by spammers or scammers trying to send misleading or harmful messages" and it started to reject messages from email addresses which used "suspicious combinations of letters that could be misleading".

There's a help center article that explains more about this feature. "If you see a warning that someone might be trying to trick you by using similar-looking characters in the email addresses of a message, you should take a close look at the sender's email address and the addresses of anyone else the email has been sent to. The addresses might be different than they seem. Sometimes, the difference is easy to spot if you look carefully. For example, someone might use a Greek character ('Σ') for the Latin character 'E'. In other situations, it's impossible to detect the difference. For instance, the Greek character ('ο') looks exactly the same as the Latin character 'o'."