problems with Email Volunteers feature

Greetings!

I’m using your plug-in to manage volunteers for our local quilting guild and everything seems to be working fine with one exception. When I try to use the Email Volunteer feature, I get an error that “The WordPress Mail function reported a problem sending one or more emails.” I should mention the mailing feature when signing up for an opportunity works just fine, as does the contact us form, etc, etc. I am using the WP Mail SMTP with Gmail as the mailer. The below error is fed back from WP Mail SMTP.

Do you have any ideas or things I should try? Any additional info I can provide? This plug-in is practically perfect for our needs and I’d love to get it working 100%.

Thank you!

Mailer: Gmail
{
“error”: {
“errors”: [
{
“domain”: “global”,
“reason”: “invalidArgument”,
“message”: “Invalid To header”
}
],
“code”: 400,
“message”: “Invalid To header”
}
}