You probably have seen news that Apple is phasing out 32bit apps. Starting with 10.13.4 there will be a scary sounding warning "Mail Archiver X is not optimized for your Mac. The developer of this app needs to update it to improve it's compatibility." This will happen only the first time you start up Mail Archiver X.
The next version 4.3 will be 64bit and will run on 10.13.4 and 10.14 without warning.
What is also coming for 4.3:
- faster archiving from Mail
- writing mails to Filemaker is more reliable
- checking duplicate mails for Filemaker/Evernote/pdf is way faster
- better handling for 2 monitors
- better authorization for Gmail
- and lots of more stuff