If the error message "Your message was sent, but an error occurred during post-processing. The reported error was "Failed to append to mbox:///home/user/.local/share/evolution/mail/local#Sent: Cannot get folder 'Sent': folder does not exist. Appending to local 'Sent' folder instead."." is shown, you can fix the problem by editing your default folder settings under Edit ▸ Preferences ▸ Mail Accounts ▸ Edit ▸ Defaults ▸ Folder for sent messages.

This bug is fixed in Evolution versions higher than 3.0.2 (stable series) and 3.1.3 (unstable development series). If this is not the case, please add a comment to the corresponding bug report.