Use a Message Filter to tag the message, based on the sender.

Sorry don't understand. Could you give more precise instructions please?

I don't see that I'd want to tag messages by hand. I'd expect it to be automatic, and for that I'd use a filter. So we can use a filter to automatically apply tags to incoming messages.

Tools|Message Filters

New...

"from" "is" ... (you could use "contains" or "ends with" instead of "is")

In the actions part,

"tag message" and select an appropriate tag.

It's not as neat as your suggested solution, since with your approach you'd set a tag against a contact and the job would be done. Here you need to create a filter for each sender of interest, and look up their email address to enter it into the filter. But you could easily cover multiple associated addresses (say, all at one business) with one filter (possibly using mulitple rules), whereas with tagging Contacts, you'd need each sender to be added to your address book and individually tagged. Your method falls over if a new sender appears and you don't yet have them in your address book.