Message Boards email notification's 'to address' uses 'from address' instead.
When you go to Content&Data → Message Boards → Configuration → Email from and set a valid email address, Message Boards will use this as a 'to address' as well by default.
Liferay DXP 7.2
By default, the notifications are sent to a mailing list. However, if no 'To address' for the list is set, it will set that address from the 'From address.'
In order for Liferay to send the notifications to each subscriber individually and not to a mailing list, there is a possibility to set message.boards.email.bulk to false in portal-ext.properties (it is true by default).