> Now they must remember which address to use for what.

I will be introducing the address/contact book feature for each user, wherein users can save the port address with whatever name they feel comfortable. So, users just need to send a message to someone from the address book (if already added), this way users don't need to remember exact port address, rather just refer the address book.

> this solution requires all users to use the same platform

I think except email, most of the solutions will require all users to use specific platforms.