The output of mu [ <https://guix.gnu.org/en/packages/mu-1.6.10/> ] (and thus, by extension, mu4e in Emacs) does not correctly handle characters outside of a certain range (which confuses Emacs to no end). This behaviour occurs both in mu4e and when using mu on the commandline.

For instance, subject lines including the 🎉 emoji ("celebration") do not show the emoji, but rather a backslashed \360. Similar issues occur with, for instance, devanagari characters (Hindi etc.).

When using mu/mu4e on non-Guix systems, this issue does not occur. (I.e. both in mu4e and in mu on the commandline these characters are correctly displayed.)

Since mu depends on xapian, it's possible the issue could also be with Guix's xapian package.