お世話になってます vs お疲れ様です I work in Company A, and mailing within Company A usually starts with: > > > We have a joint dev team with Company B, and mailing to Company B starts with: > _or_ > > _or_ An employee of Company C is on loan to our Company A. Which greeting should I use when mailing to Company C employee?

Both are possible. To put it simply, looks less respectful but more friendly, whereas looks more formal and respectful. Which to use would depend on your character/preference, the custom of your company, and the actual relationship between you and the person in Company C. Which describes the person C better, your colleague, or your client?

If you're unsure, it's always safe to get away with .