This morning I spotted a message that was flagged as suspicious by Mail Aegis from a well known Cyber Security company - that was pretty unusual, especially when I saw that it had been flagged as containing zero-width spaces. Upon reading the message it was immediately apparent as to why this rule had triggered - it referenced this article ( https://isc.sans.edu/diary/An+unusual+shy+zwasp+phishing/31626/?is=dc8443240cfd5d91e014cdfac5cd6fd93d12ee66d3e23affe2dd4845ca06d8a2 ) regarding zero-width space usage in phishing emails to trick content filters…
This isn't a common attack technique, but it has been in use for about 15 years, but Mail Aegis ( cyberwarden.io/services/aegis ) is certainly protecting our customers against it.

Read the article linked above - it's quite informative on the attack, and how those not protected by Mail Aegis can identify the attack (if they're aware of it, and are suspicious of the message received)