You could create two custom meta fields for your custom post type ‘letter’. With the plugin Advanced Custom Fields, this can be done in a minute: https://www.advancedcustomfields.com/resources/taxonomy/
With one field you choose your sender, with the other you chooce your receiver.
You can also create your fields by hand, but even the wordpress docs tell you that this plugins can be used: https://wordpress.org/support/article/custom-fields/
Meta Box Plugin: https://wordpress.org/plugins/meta-box/
Picklist: https://wordpress.org/plugins/piklist/
Advanced Custom Fields: https://wordpress.org/plugins/advanced-custom-fields/
Advanced Custom Fields is awesome! It really gives you the tools to turn your wordpress page in an easy to use but complex content management system.
There are a lot of build in fuctions to use and methods to save data, get data, show data and hook into before save and much more.