Hi
red97,
sometimes it happens that recommenders need to use their personal email addresses (e.g. they are unemployed at the time of your application, they are away with no access to their work emails, etc...).
So, as long as the school's online application system accepts personal email addresses (e.g. gmail, yahoo, hotmail...) and recommenders can get the link to the recommendation system, you should be good.
Most of the time, there is a question asking your recommender to tell how you two know each other and since when. There, your recommender could include a note explaining why he is not using his work email address and could welcome the school to call him directly by phone in case of any concerns.
You could also explain the same in the optional essay.
In any case, this shouldn't affect your application in terms of credibility.
I hope this helps.
Thank you very much.
Warm regards
Alex