Validate email address fields

[Dan-eh3]

When configuring SMTP settings, the Sender and Recipient address fields are not validated. Users can input an impreperly formatted address without warning or validation.

Please improve these fields to validate that an email address is inputted, in example name@domain.com

[julsemaan]

This needs to go in maintenance/9.0 and maintenance/9.1

[satkunas]

@julsemaan alternative adjustments will be needed since the admin was reworked after these releases. Proceed anyway?

[julsemaan]

Let's leave it as-is for now since these are fairly old releases by now