1) Sometimes the filter allows to send emails to email addresses even though that user isn't created in the JIRA instance. On the other hand, it is not allowed to send to that same user.

2) Most of the mails sent via the Share Button, contains a link to and empty issue search query - which then doesn't make sense.

In a nutshell, it does sometimes allow to send shares to non-JIRA users. Other times only JIRA users. The inconsistency causes confusion to the JIRA users.

Perhaps the developers can come up with a fix to this or even a global feature to disable the Share button.