When I'm contacting a colleague for the first time and we're communicating online, I can use はじめまして as part of my greeting. Likewise, if I'm meeting someone for the first time and it is an in-person meeting, I can say はじめまして during my introduction.
However, if I talk with someone online and then we meet in person for the first time, is はじめまして still appropriate? Or is there some other typical greeting for this situation?