Canada Post says it does not have the legal right to refuse to deliver the mail, despite a number of complaints about graphic anti-abortion flyers targeting Liberal Leader Justin Trudeau.
The flyers first surfaced last spring, when residents of Ottawa and Saskatoon, among other cities, complained about the pamphlets' disturbing images, which included aborted fetuses.
This week, the flyers reappeared in Toronto mailboxes, but were concealed in plain envelopes marked with the words "Important Election Information."
On Thursday, a Canada Post spokesperson said there is little the Crown corporation can do.
"We do not have the legal right to refuse delivery of a mail item because we or other people object to its content," Anick Losier said in a statement to CBC News.
"Anyone who has concerns about the content should either contact the publisher or simply dispose of it."
In June, a Saskatoon father filed a complaint with police after one of the pamphlets arrived at his home.
"As far as I'm concerned, this amounts to child abuse," said Robert Rudachyk. "My next door neighbour's granddaughter got the mail yesterday and when she saw it she was so traumatized, she vomited on the floor."
The group responsible for sending them, the Canadian Centre for Bio-Ethical Reform (CCBR), defended sending the pamphlets, which criticize Trudeau for his pro-choice views. Cameron Cote said children are not the targeted audience.
"Abortion itself is very offensive," Cote, Western activism and outreach director for the CCBR, said in June. "If we're offended by the image, possibly, we should think a little more about the action that's being done."
The group is targeting new ridings and swing ridings ahead of the upcoming federal election.