Hello,
the body of emails can be modified by editing the FreeMarker templates inside the Data Dictionary/Mail Templates folder.
Changing the subjects actually requires a restart as you need to provide I18n message bundles with specific keys to override the default. For an invitation, the message key is "invitation.invitesender.email.subject" and for activities "activities.feed.notifier.email.subject". Other mail sending actions may not use I18n message bundle keys, so can not be altered by simple configuration.
Regards
Axel