Send to Mail Recipient in Thunar always gives me this error.

I’m sure I have Googled for a solution this many times before but never found one.  Today I did.  It’s simply that exo depends on perl-uri to complete this action.  It’s even an optdepend in Arch:

 Optional Deps : perl-uri: for mail-compose helper script

See, so

 pacman -S perl-uri

fixes it with no further effort.