Currently, the latest version of Exim available on 18.04LTS is 4.90. However, several exploits have been found, requiring upgrade to 4.92.3. My understanding is that 18.04LTS is still within the maintenance life cycle.
Are there plans to update Exim anymore in 18.04LTS?
Thanks
https://ubuntu.com/security/CVE-2019-13917
https://ubuntu.com/security/CVE-2019-15846
Ubuntu fixes security issues in older versions of packages it distributes. As you can see in the links above, the most severe exim4 vulnerabilities have been patched in Ubuntu in 4.90.1-1.
https://www.cvedetails.com/vulnerability-list/vendor_id-10919/product_id-19563/Exim-Exim.html