The wording is a bit verbose, but your problem is here:

A recipient must be specified.

Be sure that you are always providing an email address when sending an email.  This is a common mistake when using a SmartObjects in the "To:" field, then a user is encountered that doesn't have an email address.  This can also happen if you are sending to a list of email addresses and you have an extra semi-colon.  For example, consider the following strings:

foo@bar.com;bar@baz.com;

foo@bar.com;;bar@baz.com

Sending an email to either of these would result in the exception you are seeing.  The first is invalid because the trailing semicolon gets interpretted as an empty address at the end.  The second is invalid because the two semicolons gets interpretted as an empty address in the middle.

I had the similar problem in our project.

I found if you add ";" at the end of last email address it will cause an issue. Also if you add Multiple ";" in between 2 email address it will also fail and gives you similar error.