This probably is not useful information after the fact, but one mitigating control for email server data loss is to use something like Thunderbird assuming the

This probably is not useful information after the fact, but one mitigating control for email server data loss is to use something like Thunderbird assuming the service supports IMAPS. Thunderbird can either pull all the emails locally, or pull them locally and leave a copy on the server. Thunderbird handles de-duplication by message ID. If the goal is to keep the emails on the server, then one can use Thunderbird to push the missing emails back up to the server. There are other command line tools that can automate this process and even keep the backup copies of the emails on cloud servers if one wished. This probably isn't super useful after the fact but something to consider to mitigate against free services losing data in the future.