Oh no! A few people reported this crash today, I'll be fixing it hopefully tomorrow!

You can reset your cache by running these this in Terminal: rm ~/Library/Containers/com.mimestream.Mimestream/Data/Library/Application\ Support/Mimestream/Mimestream*

rm -rf ~/Library/Containers/com.mimestream.Mimestream/Data/Library/Application\ Support/Mimestream/Attachments