Heat 2 1/4 cups of water to 208°F. If you have an electric kettle with a temperature setting, great. If not, bring the water to a boil in a saucepan, kill the heat and count to 20. Pour the water over the teabags into a quart vessel and steep 3 minutes before removing the bags. (Do not wring out or squeeze the bags.)
Add the sugar to the hot tea and stir to dissolve. Then add the port, rum, Allspice Dram, and lemon juice. Cool to room temperature, about 20 minutes.
Pour the milk into a glass pitcher and then follow with the tea mixture. (Yes, it will curdle.) Park on counter for 1 hour. Do not shake, stir or molest in any way. Just let the magic happen. Oh, and make sure the tea goes into the milk, not the other way around.
Line a large, fine-mesh sieve with a commercial-sized coffee filter (we like those made for 1.5-gallon machines) and strain the mixture into a wide, 4-quart container. Once completely filtered, typically about 1 hour, transfer to a quart jar or pitcher and chill to serve. (See note.)
Store tightly sealed in the refrigerator for up to 2 months.