EMAIL DELIVERY ERROR

Replies: 0

Hi, this message is showing in my wp-admin area:

Mailer: Gmail
{
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "invalidArgument",
    "message": "Invalid to header"
   }
  ],
  "code": 400,
  "message": "Invalid to header"
 }
}

Can someone help? Thanks!