Only one program at a time can bind to a specific port, so if postfix has attached to that port (ie usual setup) then you don't need to worry.
Postfix if an SMTP server, so it won't give up that port unless you kill postfix.
Note that a port isn't really 'open' unless a program is attached to it. Its a bit of a misnomer, its not like a porthole on a ship...
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