If WHMCS emails are not sent out,

are you having setup daily cronjobs (Setup / Automation Settings)

another thing is to check is (Utilities / Logs / Email message log)

if some email is NOT sent out, check its template (Setup / Email Templates) if it do not contains some bad code.

Example i used code like

{if $variable_name eq "something"}print some text{/if}

but it should have been single quotation marks, not double (")