I can't tell which email system or app but here, I use Thunderbird across machines without any trouble. However even with no troubles, it still means I backup what I can't lose. In Thunderbird it's widely documented how to backup and restore emails.
I had my email, including my Pop email, wiped out the end of January without prior notification/explanation. Tried rebuild to no avail. Some messages appeared in documents but not many. Now email is deleting emails and only leaving headers. Is anyone else having these difficulties?