I use WHMCS with EZPZ and I am not having any problems. Maybe that's because I use PHP Mail instead of SMTP. What's the reason you want to use SMTP instead of PHPMail?
Ironically, when I swapped to EZPZ they advised me to swap the setting from php (which I had been using my previous provider without any issue) to SMTP
it worked for two days, then stopped again, then I swapped it back and it has been fine since
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