You will need to test your email account, outside of ubb and within ubb.
It could be blacklisted or the mail service is not running.
In ubb you elect what email account sends the email.
Control Panel » Primary Settings
Board Email Address:
All emails being sent out will come from this address.
Also if you expand the advanced option you will see:
Use specified SMTP Server:
Normally this is controlled by php.ini. However if you need to send mail through another SMTP server and cannot do this by editing php.ini, you can specify it here.
It is highly unlikely that a email issue is a bug in version 7.5.7.
It would be you entered your email or smtp server info incorrectly.
Or your email host has issues.
Or your email was blacklisted and can't be delivered.