Poppy Carter was found guilty of stalking Carly Bradbury. St Albans Crown Court heard she poured paint and a corrosive liquid over her work van and sent anonymous cards depicting a cat watching a goldfish.
Passing sentence on Friday, Recorder Leslie Cuthbert called Carter's actions "despicable, evil and vindictive."
"This conduct was intended to maximise her distress," he told her.
She was told a custodial sentence was justified for what she had done but that the jail term was suspended for two years, the BBC reported.
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During the trial in July the jury heard the stalking happened over seven months in 2016.
Carter employed a private detective to find out where her ex-boyfriend, Alex Roberts, had moved to with Bradbury.
The following month a dead mouse arrived "belly up" in a Jiffy bag at the home in Watford.
Prosecutor Daniel Wright said her actions had "amounted to the stalking of Bradbury, causing her alarm" and the mouse was an "extremely distressing and frightening thing to find".
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