The poem opens with a blunt, almost shrugging claim: "Rot is rot." Decay is decay, whether it comes as "full satisfaction" or as the dull stretch of an office afternoon. The comparison is odd and a little funny. It puts the fulfilled life and the bored, ordinary one on the same level, as if both end up in the same place.
From there the speaker offers a strange piece of advice: write letters to the dead and the mad. The reason is the last line, that they live "all within / a door or two." The dead and the mad aren't far away. They're close, maybe next door, maybe just past the edge of our everyday sanity.
The mood is dry, wry, and quietly unsettled. The speaker doesn't sound panicked about mortality or madness. Instead there's a calm, deadpan acceptance, and even a hint of preference. If everything rots anyway, the people who have stepped outside ordinary life may be better company than the office.