The lady had jilted the schoolmaster.
Whether this explanation came from the post-office, who shall say?
But so long as he lived the schoolmaster was twitted about the lady who threw him over.

He took his revenge in two ways.

He wrote and posted letters exceedingly abusive of the postmistress.

The matter might be libellous; but then, as he pointed out, she would incriminate herself if she "brought him up" about it.

Probably Lizzie felt his other insult more.