Let's Encrypt on Exchange 2013

Preparation

Go to https://www.win-acme.com/ and download the latest release and unzip it to a directory of your choice.

Open on your firewall port 80

Make sure that on your IIS you have a default site on port 80

Execution

Open a CMD with Administrator privileges 

run: 

wacs.exe --target manual --host mail.yourdomain.com,autodiscover.yourdomain.com --certificatestore My --acl-fullcontrol "network service,administrators" --installation iis,script --installationsiteid 1 --script "./Scripts/ImportExchange.ps1" --scriptparameters "'{CertThumbprint}' 'IIS,SMTP,IMAP' 1 '{CacheFile}' '{CachePassword}' '{CertFriendlyName}'"

Close Port 80

That's it