evolution MUA (mail user agent; or email program) is available for Ubuntu-MATE 21.04 users, but it's general practice for only security patches to be back-ported to older releases, not newer features/programs.
If you want the latest software, the non-LTS upgrade path is generally the best (ie. 20.04 upgraded to 20.10, then 21.04 where it's available now, etc.)
A CLI query on my boxes shows available
evolution | 3.36.1-2 | focal/universe | source, amd64, arm64, armhf, ppc64el, riscv64, s390x
evolution | 3.36.5-0ubuntu1 | focal-updates/universe | source, amd64, arm64, armhf, ppc64el, riscv64, s390x
evolution | 3.40.0-1 | hirsute/universe | source, amd64, arm64, armhf, ppc64el, riscv64, s390x
ie. if you had upgraded to the latest Ubuntu-MATE, you'd be using the
evolution you seek.
Other 3rd party sources (PPAs etc) may provide later packages, or other users here may offer other alternatives (eg. is a snap package available? go look in the snapcraft store, a flatpak etc), but note adding 3rd party sources can complicate release-upgrade time (ie. when you want to upgrade to Ubuntu-MATE 22.04 LTS when the time comes).