Add email address verification on user creation and when sending mail



in reference to the following git thread:

It would be great if a email address validity check could be added when creating users or when sending emails via zammad and writing addresses into the TO/CC fields.
The issue is that oftentimes when moving customers manually to zammad after a phone call or similar e-mail addresses are quickly typed or copy pasted from other e-mails etc. which can lead to invalid mail addresses due to typos or unsupported characters.

Currently Zammad only checks if the string has a top level domain after the @ and that is it.
This leads to the situation that a user can be created with email address: examplename@exampledomain.com1312151
Adding an actual email validity check like e.g. the web form has would avoid unwanted follow up issues.