User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0 Build ID: 20180516032328 Steps to reproduce: 1] Used search bar Ctrl+K to search for messages containing fo@foobar.com 2] Used search bar Ctrl+K to search for messages containing f@foobar.com Actual results: Search returned erroneous results containing a-Z@foobar.com i.e., (a-Z) being any number of characters and any characters, e.g. random@foobar.com, another@foobar.com, etc. Expected results: 1] only messages containing fo@foobar.com should have been returned 2] only messages containing f@foobar.com should have been returned As soon as the prefix is 3 characters or more (e.g., foo@foobar.com), this bug disappears.

@ is considered to be punctuation, and the string before @ in your examples is less than 3 characters. Because global search search terms are less than three characters in length, your "f" and "fo" are thrown away when when indexing the data. This is essentially bug 523183