Can you share a sample of the code attempting to send the messages?

I have never encountered this myself when sending emails via php but some googling found a number of people reporting similar issues and the primary suggestions I found there were:

• Escape the hyphen: digital\-ocean@example.org
• Wrap the email in quotes: "digital-ocean@example.org"
• use angle brackets: <digital-ocean@example.org>

Here is the script. As you can see I'm using PHP Mailer

require 'PHPMailer.php';

$name = $_POST['name'];
$email = $_POST['email'];
$tel = $_POST['tel'];
$where = $_POST['where'];
$message = $_POST['message'];
$to = "";

if ($where == "one") {
  $to = "email1@example.com";
} elseif ($where == "two") {
  $to = "email-2@example.com";
}

$mail = new PHPMailer;

$mail->isHTML(true);
$mail->From = 'form@example.com';
$mail->FromName = 'Website Contact Form';
$mail->addAddress($to);
$mail->Subject = "Contact Form Submission";
$mail->Body = "<b>Name:</b> " . $name . "<br><br><b>Email:</b> " . $email . "<br><br><b>Telephone:</b> " . $tel . "<br><br><b>Where They Are:</b> " . $where . "<br><br><b>Message:</b> " . $message; 

$mail->send()

Thanks.

Looking at PHPMailer's source code, it appears to encode a recipient email address only if there is a name. Try passing a name to addAddress(), does that work?

$mail->addAddress($to, 'Name here');

I tried that and it didn't work. I think it says somewhere that the name isn't required anyway.

Now I can't send emails at all. I get this line in my mail.log

Jun  2 16:33:54 McCarthee sm-mta[22590]: t52KXqS3022588: t52KXsS3022590: DSN: Service unavailable