I remember I did in the original Gold version by having another much lower leveled Pokemon holding the mail. Does it work in HG/SS?
Yes, You Can Keep The Stupid Bird. You Just Dont Give it Back. You Steal It and Cant get rid of the mail or put it in the pc, without giving it away. I Myself Kept It For A While:) Unfortunatly, you can't get the TM if you keep it.
Give mail to hold ANY other Pokémon - you will lose ONLY mail and keep all the Pokémon.