Early Konoha, Madara researches Senju tradition in order to send Tobirama what basically amounts to hate mail in flower form— but the Senju flowers language is updated every time there's a new Mokuton user, and so his information is incredibly out of date.
What he actually manages to send Tobirama is a very romantic bouquet that speaks of years of yearning, wishes for passion, and forbidden love
Tobirama, who is allergic to normal human communication, very coldly (shyly, in that odd way of his) replies with his own bouquet of quiet, slow love, a confession given in secret, and willingness to give chance to new beginnings.
Only to Madara it reads as returning his "fuck you" bouquet, and he of course replies in kind
Tobirama thinks they're having a forbidden, secret, slow burn romance
Madara thinks they're sending eachother hate mail (and if the Senju is getting more oddly tolerable these days,, we'll, he's not going to stop with his spiteful flowers. That'd mean admitting his loss!)