why my Xenforo script is not sending email when I use PHP built-in mail system?

which control panel did you use?
If cPanel then check email properly work or not.

configure it on SMTP

also check for blocked ports

meroforum said:

I tried but the system says unable to connect to your host smpt.gamil.com.
if you know please guide Me

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try with your domain: mail.site.com

did you check server error log in xenforo admin panel

meroforum said:

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use paid mail service.

This site will help diagnose problems, mxtoolbox.com ... but my suggestion is to rather use cpanels default mail server, via smtp. there is just so many variables to factor in with mail, and headers, and dkim, dns settings and so on, its honestly not worth the headache. And ofc ensure u have php modules open-ssl, imap, etc. enabled.

meroforum said:

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Might wanna disable that Andy/SimilarThread addon, as there is possibility its causing execution to terminate prematurely.. and resolve the duplicate account check thing..

meroforum said:

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try disabling that add on and also check "duplicate account check error"