sorry, but its not nonsens.
at first, we've to ask: did we talk about the basic php-function mail(), that use the sendmail()-function from the server?
next step: do you use the shortest way like this -> mail('example@example.com', 'MySubject', $myMessage); or the extended function with additional header-informations with From..., Reply-toAdress, X-Mailer...
a lot of mailservers detect a send-mail as spam, if there are missing the FROM && Replay-Adress, other servers blocked the port 25 complete and so it is possible, that you can send a phpmail to me with success, but my mailserver detect it as spam
for example: i'm using the german provider Strato for any of my webprojects. With the new GDPR (General Data Protection Regulation) the provider stops the simple php-mail and blocked the port 25, now, a couple of years later, i can use it again, if i use the additional headers.
at 1&1 (IONOS), another german provider, i can use only SMTP-Mail, but it looks, that it works only for the simple webspace-account and not for a webserver.