Background
[From JEMH v3.3.95-server release notes - https://marketplace.atlassian.com/apps/4832/enterprise-mail-handler-for-jira-jemh/version-history ]
Jira unexpectedly made a breaking change in the 8.22.3 (released 23-May-2022 - https://www.atlassian.com/software/jira/download-archives ) micro release that JEMH was marked compatible with automatically. This release works around the non-longer injectable UserManager.
It since appears that this issue was back ported into Jira 8.13.21 LTS (released 23-May-2022) and also 8.20.9 LTS (released 23-May-2022).
Impact
JEMH versions earlier than 3.3.95 will not enable on Jira 8.13.21+ or 8.20.9+
Solution
A fix has been implemented in JEMH 3.3.95+ (Server and DC editions) which will resolve this issue. Upgrade to get the fix - a valid license is required to use such updates.
Lapsed Server licenses & use of upgrades
Server licenses lapsed by less than a year need to have maintenance extended. Server licenses lapsed by more than 1 year require a repurchase. Contact sales@thepluginpeople.com referring your ServerID as needed.