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I am using PHP mail to send emails but the emails aren't showing up in the inbox and there are no errors in system mail logs or errors.log. When I try some disposable email address to register it's working fine. I am guessing it's some kind of filter that is blocking. This is what I get when I use a disposable email.

Received: from localhost.localdomain (Unknown [127.0.0.1])
	by 3674c76badb5 (Haraka/2.8.25) with ESMTP id 09333DCE-23CA-4252-B05F-EE8119779AF0.1
	envelope-from ;
	Mon, 19 Oct 2020 03:11:39 +0000
Received: from localhost.localdomain (localhost [127.0.0.1])
	by localhost.localdomain (8.15.2/8.15.2/Debian-14~deb10u1) with ESMTP id 09J3Bd4P028156
	for ; Mon, 19 Oct 2020 03:11:39 GMT
Received: (from www-data@localhost)
	by localhost.localdomain (8.15.2/8.15.2/Submit) id 09J3Bdgn028153;
	Mon, 19 Oct 2020 03:11:39 GMT
Date: Mon, 19 Oct 2020 03:11:39 GMT
Message-Id: <[email protected]>

After this, there are 'To', 'From' etc.

AFAIK, Gmail will block senders if certain criteria are not met, please refer to https://support.google.com/mail/answer/81126 .

From OP's post, I'm not sure if the mail has reached to Gmail server, meaning that the mail was rejected by Gmail server or the mail went into the Spam folder.

I'd recommend using SMTP as opposed to PHPMailer, generally speaking most providers score emails negatively if they do not have valid DKIM signatures, which PHP doesn't provide. Mailgun is a great provider that offers free outbound SMTP service up to a limit of 5,000 messages per month.