Police said the man, in his 20s, had been arrested by armed officers in Cambridge, central England, over the suspicious item sent to a property in Cricklewood in north London on Thursday.
"Specialist officers attended and made safe what was assessed to be a small improvised explosive device," they said in a statement.
The man is being questioned at a south London police station while two addresses in Cambridge are being searched. There was no risk to the local community, police said.