Archiveium is a free, open-source & self hostable email backup solution that brings together emails from different providers in a single location.
For the emails, it is a one way journey i.e. new emails get added but deletes have no effect. This ensures you always have a copy of email in Archiveium even if it gets deleted on remote!
There are a number of additional features in pipeline that are planned for future. Here are some of the noteworthy one's,