Your ISP isn't reserving the port for their own uses, they're likely filtering/blocking it to help decrease spam.. i.e: protecting others from improperly configured servers.
You might be able to contact them and ask them to remove the block.. but if you're on a residential plan, you might be in violation their AUP, TOS.. admitting you're doing so would probably be a bad idea.
If however you're on a business plan, they may be more receptive to your request.. or they might be able to handle your mail requirements themselves.
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