How do you know it's the browser? I mean, you don't get the GREEN icon (success) in Chrome?
When you get a GREEN icon, it means the PHP (X3) has successfully sent the message by the emailer method in your settings. What happens to the email after it's sent from X3, is outside of X3's control, and depends on your email settings (SMTP?, server php mailer, "from" address, SPF, DKIM, and loads of stuff that affects "spam" and may block email from being received, or blocked before the even go to "spam"). I have never seen or heard of a mail function failing on specific browsers, not as long the email actually gets sent (which must be the case if you get green icon).
I tested from here, and the mailer reports send success:
That response is from the internal PHP mailer mechanism:
$success = $mail->send();
If it reports sent success, it gets "sent" from the PHP, no matter what (no matter browser). What happens from when the email is unleashed from PHP into your server (or SMTP) mailer mechanism, is outside of PHP (and X3's) control. We use
https://github.com/PHPMailer/PHPMailer