Default is set to the hostname.domain, but sometime you might want to have something else to answer with the same as your reverseDNS. You can do one of the followings to only adjust the helo name:
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config setprop smtpd HeloHost mydomainname
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signal-event email-update
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or the following to adjust the way your server will present itself everywhere (httpd, qpsmtd...) This might trigger the generation of new ssl certificate, so use it only if you are sure this is what you want to do.