Replacing the letter O with the number 0 or the letter l with a 1 in a message or a link can mislead the user and lead into a trap set by online scammers. Danger against which Google wants to fight in Gmail: the company announced via a blog note that it will now look into "suspicious character combinations" in the emails received by Gmail users.
Google is working on this in agreement with the Unicode Consortium , applying a level of "very small" detection to not penalize legitimate users. Misleading elements were identified, and where their presence is detected within an email, it is always "rejected by mail." It is not clear whether the messages are sent in spam where they do not appear at all insofar as zero risk does not exist, it is not impossible that a legitimate message goes out the window with such a system.
Gmail starts applying this filtering system this week for the user, there is nothing to do, since the process takes place in the background.