Shamim's Plugins

It depends on where you want to show that button.
Add following code in your theme’s (child theme’s if you are using) functions.php

add_action( 'fep_display_after_parent_message', function(){
	if ( fep_is_group_message( fep_get_the_id() ) ) {
		echo '<a href="' . fep_query_url( 'newmessage', array( 'fep_to' => fep_get_userdata( fep_get_message_field( 'mgs_author' ), 'user_nicename', 'id' ), 'message_title' => fep_get_the_title() ) ) . '" class="fep-button">Reply to Sender</a>';
	}
});

It will show a button to reply to sender bellow your main message content if that is a group message.

1. Please recheck if you set “Front End PM Page” in Front End PM PRO > Settings > General
2. If you deactivate that plugin, does it works?

Let me know.

Please go to Dashboard > Front End PM PRO > All Messages and select your messages and bulk delete.

Your code is correct except last semi-colon.

I see you are using a custom plugin for this plugin. In that plugin you are using get_current_screen() function without checking if that function exists or not.

Please go to custom-fepm-config.php around line 56 and add function_exists( 'get_current_screen' ) check before that.

How to add debug code?

You will found this option in Front-end settings (from where you send your messages)

1. Other checks have satisfactory results?
2. So only live site is not sending emails? normal emails are being sent? eg. registration email?
3. Did you do a conflict check? Eg. deactivate all plugins and activate one of default theme (eg. tweenty*) ?

Hi Pavlo
Please follow https://www.shamimsplugins.com/docs/front-end-pm-pro/troubleshoot/email-notification-not-receiving/
If still not working please create a separate topic so that i can support you better.