Well, you could say work email is cunning, Toby.
Already he has people like you telling the OP it must be a work contact, not a prostitute, because it's work email.
I can well imagine that playing out in the confrontation between partners! "of course it's a work colleague, look it is work email"
Also, I'd guess that suspicious partners (or non suspicious ones!) are far less likely to check work mails.
People use work email for dubious purposes all the time, ask any IT department.
You may think it's a stupid thing to do, but most of the women on here discovering prostitutes or affairs seem to do so because their partner slipped up, got lazy, was stupid. Secret phones left out, Facebook left logged in.
FWIW, my XH kept his old phone, riddled with prostitute contact texts (deleted content but not date/number). So when mine died, I borrowed his. He had forgotten or thought the deletion of content enough. Obviously I'm glad of his stupidity!!
Men who cheat for years get complacent.
I am not at all surprised OP's husband used work email.