How do you change the port number?

ive a dating website which should contact members when they receive messages/winks but keep getting my email box flooded with email sending failings. My website host says the problem is i need to change the port number. Where do i find the port number and how do i change it. Am quite a novice to this so a fairly detailed reply would be very helpful
How do you change the port number?
i did not know bottle of port had numbers

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