You're trying to send email using a gmail address as your "from address", right? That's always going to cause issues unless you are sending via Google's mail servers, because to everyone else, it looks like you are trying to send spam.
Use an email address associated with your website domain as your from address and set up SPF/DKIM/etc - make sure it's a valid address which can receive bounced email notifications for processing by XenForo.
Even better, use a 3rd party ESP like SparkPost, Mailgun, Amazon SES, Postmark, etc.