Here is the error we're getting when the forum tries to send an email to Zoho:
2022-12-06 00:20:49 1p2LhX-0003r3-IX ** <removed>@fasouza.eu.org R=dkim_lookuphost T=dkim_remote_smtp H=mx.zoho.com [136.143.191.44] X=TLS1.2:ECDHE-RSA-AES256-SHA:256 CV=yes: SMTP error from remote mail server after RCPT TO:<<removed>@fasouza.eu.org>: 541 5.7.1 Mail rejected due to antispam policy
We're not listed in any of these 82 blacklists https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3a65.19.141.66&run=toolpage so Zoho must have their own blacklist or something. Sometimes these error messages have a link to their website where you can request your IP to be delisted, but Zoho doesn't provide that apparently. You'd have to contact Zoho support and ask them why the email isn't being delivered.
Perhaps you could change your Plesk email to match your forum email instead?