Shamim's Plugins

Add following code in your theme’s (child theme’s if any) functions.php

add_filter( 'fep_form_submit_button', function( $button, $where ){
	if( 'reply' == $where ){
		$button = '<button type="submit" class="fep-button" name="fep_action" value="'. esc_attr( $where ) .'">SEND</button>';
	}
	return $button;
}, 10, 2 );

“No Parent” means that message itself is a parent message. It does not indicate that message has responded or not.

Yes.
Please use any other Mailer in your SMTP plugin setup. If possible use your server SMTP setup.
I see you have “From Email” setup as same domain email. Use this email details to send email.

Please use ftp/file manager then go to your website wp-content/plugins directory and remove shamim-email-log folder.

If you find any difficulty you can give me FTP access.

It seems your sendgrid ip is blacklisted by spamcop.
Can i temporarily deactivate your SMTP plugin and install a custom plugin which will log every email in your server instead of actually send email?

Why those email not sending?

It is possible but require custom coded.
If you need that code from me you can hire me using https://www.shamimsplugins.com/hire/

We need to pass post id along with message. Then save that post id when message save. Again show that post in view message.
It needs to be custom coded.
If you want this feature you can hire me using https://www.shamimsplugins.com/hire/