[xenous]

Hello,

My solution will only work if your SMTP is working fine.

I am using
https://wordpress.org/plugins/wp-mail-smtp/
with mandrill and my forums never has issues. I have 10k+ active members and no hiccups till now

[xenous]

Check your Forum Settings for topics and replies per page section. Either set right or wrong save the page.

[xenous]

you can configure the noreply email simply with:

add_filter('bbp_get_do_not_reply_address','my_bbp_no_reply_email');
function no_reply_email(){
    $email = 'noreply@yourdomain.com';
    return $email;
}

bbPress uses BCC for sending subscription emails and the noreply email is used as to field. This is for better hardware (cpu and memory) usage as if you were to loop each user subscribed to content it would be troublesome.
you can change that default to email address as well with :

add_filter('bbp_subscription_to_email','my_bbp_subscription_to_email');
function my_bbp_subscription_to_email(){
    $email = 'noreply@yourdomain.com'; // any email you want
    return $email;
}

I hope this helps.