Within the /etc/exim4 directory run:

openssl req -x509 -sha256 -days 9000 -nodes -newkey rsa:4096 -keyout exim.key -out exim.crt

Shows

Generating a 4096 bit RSA private key
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writing new private key to 'exim.key'
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You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
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Country Name (2 letter code) [AU]:UK
State or Province Name (full name) [Some-State]:Jersey
Locality Name (eg, city) []:St. Helier 
Organization Name (eg, company) [Internet Widgits Pty Ltd]:ShareWiz
Organizational Unit Name (eg, section) []:Tech
Common Name (e.g. server FQDN or YOUR name) []:mail.sharewiz.net
Email Address []:admin@sharewiz.net