I have seen this error as well, and found the problem to be the email
address having a leading 'space'.
For example, I was passing an email address like this: " Bob Smith
<firstname.lastname@example.org>" and was getting the error.
As soon as I stripped the space away with '.strip' before I assigned it
to 'recipients', the error went away.
Not sure which part of the code (gem or ruby) was choking on the
whitespace. I didn't investigate further. I am using Action::Mailer