Based on your description, I understand that you want to track emails sent from an external domain to an internal domain recipient a year ago.
Unfortunately, mail tracking can only be tracked for a maximum of 90 days, and emails from a year ago cannot be traced.
Microsoft official documentation also clearly states that the time frame for email tracking is 90 days.
More information can be found Message trace in the new EAC in Exchange Online | Microsoft Learn
As you said, the maximum search time frame for Start-HistoricalSearch is also 90 days.
More information can be found Start-HistoricalSearch (ExchangePowerShell) | Microsoft Learn
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