It's a part of a Dilbert's strip you can find here:
I understand every word. But not the joke as a whole. Can you explain the irony?
Karma states that both our evil and our good acts return to us - impartial fate guides us in ways that reward or punish us according to our deeds.
The unresponsive guy is coming across Dilbert during a meeting. Dilbert berates him publicly, and uses the guy's bad karma (resulting from ignoring his emails) as an excuse to do so without any remorse.
Since Karma is a neutral, impartial force of the universe, the irony is in performing a completely conscious, subjective, petty act of revenge and claiming it's not you who is to be blamed for this act, it's the karma.
"Say hello to karma" means "Your past has caught you up" or "You have reached the point where you cannot escape facing up to your past actions or lack of actions".
Dilbert is stating the obvious.