The ports used by Trojans and other malware tend to vary greatly. In fact there are so many that it would be virtually impossible to list them all here. Your antivirus vendor should be able to help you determine whether a connection has been made by an e-mail program or Trojan trying to enter your machine.
If you suspect that someone or something is connected to your PC without your consent, you should immediately disconnect. Another solution, although not without risks, is to try to enter the machine that is trying to attack you. However, a safer option is just to disconnect, scan your entire system with your antivirus and then reconnect.
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