Well, if you didn't installed anything or changed your configuration, the issue cannot come from our end.
You probably changed something in the main configuration or started using something special.
To me, it can only come from the email itself, try to save our custom email and replace it with the original one, it will help us to know if the issue in indeed coming from your modifications.
Also, did you modified anything in the custom fields? It may come from there since addresses are custom fields.