Why when someone register on my site, they don’...

Solution 1: Make sure IMAP is enabled on your hosting configuration -> https://diaboliquedesign.com/screenshots/3/screenshot_at_12-52-09.png

Solution 2: Add following code snippet in the theme functions.php file:

add_filter('bp_email_use_wp_mail', '__return_true');

Solution 3: Probably the IP address of your server is blacklisted. You can check it here – http://whatismyipaddress.com/blacklist-check There are 2 solutions: buy unique IP address or use some external e-mail like Gmail, you can do it using this plugin – https://wordpress.org/plugins/wp-mail-smtp/