The Exim MTA (Mail Transfer Agent, aka mailer) recently added a mandatory new security feature to 'taint' data taken directly from the outside world, with the goal of reducing the potential for future issues like CVE-2019-13917. Things that are tainted include not just obvious things like the contents of message headers, but also slightly less obvious things like the source and especially destination addresses of messages, both their domains and their local parts. There are many common uses of now-tainted data in many parts of delivering messages; for example, writing mail to '/var/mail/$local_part' involves use of tainted data (even if you've verified that the local address exists as a user). In order to still be usable, Exim supports a variety of methods to generate untainted versions of this tainted data.

Exim introduced tainting in Exim 4.93, released in December of 2019. Unfortunately this version's support for tainting is flawed, and part of the flaws are that a significant number of methods of de-tainting data don't work. It's probably possible to craft an Exim 4.93 configuration that works properly with tainted data, but it is going to be a very ugly and artificial configuration. Exim 4.94 improves the situation significantly, but even then apparently you should use it with additional fixes.

Ubuntu 20.04 ships a somewhat patched version of Exim 4.93, but it has significant de-tainting flaws and limitations which mean that you don't want to use it in its current state. As is normal and traditional, there's essentially no prospect that Ubuntu will update to Exim 4.94+ over the lifetime of Ubuntu 20.04; what we have today in 20.04 is what we get. As a result, if you use Exim on Ubuntu, I think that you should skip 20.04. Run your Exim machines on 18.04 LTS until 22.04 LTS comes out with a hopefully much better version of Exim.

If you absolutely must run Ubuntu 20.04 with some version of Exim, I don't recommend building your own from upstream sources because that has inherent problems. The Debian source packages for 4.94 (from testing and unstable) appear to rebuild and work fine on Ubuntu 20.04, so I'd suggest starting from them. Possibly you could even use the Debian binary packages, although I haven't tried that and would be somewhat wary.

(It's possible that someone will put together a PPA for the Debian packages rebuilt on Ubuntu 20.04. It won't be me, as we're skipping 20.04 for our Exim machines. It's also possible that someone will get the Exim 4.94 package from Ubuntu 20.10 included in the 20.04 Ubuntu Backports. Anyone can make the request, after all (but it won't be us).)