Correct way in new versions of nginx

Turn out my first answer to this question was correct at certain time, but it turned into another pitfall - to stay up to date please check Taxing rewrite pitfalls

I have been corrected by many SE users, so the credit goes to them, but more importantly, here is the correct code:

server {
       listen         80;
       server_name    my.domain.com;
       return         301 https://$server_name$request_uri;
}

server {
       listen         443 ssl;
       server_name    my.domain.com;
       # add Strict-Transport-Security to prevent man in the middle attacks
       add_header Strict-Transport-Security "max-age=31536000" always; 

       [....]
}

If I understand your situation correctly, your problem is that %{SERVER_PORT} is never equal to 443, since your server is not listening on that port. Thus, your RewriteCond %{SERVER_PORT}=443 condition will never be true.

In fact, I don't believe it is possible to do the rewrite that you intend, unless you put a server listening at port 443 just to handle the initial incoming requests that you then immediately redirect to port 444. But if you do that, you might as well not bother redirecting at all; just handle the HTTPS requests at 443.