Royal Mail has warned customers in central Scotland not to post items unless they are urgent after wildcat strikes in Glasgow and Edinburgh.
The strike action has spread to Edinburgh
It said it could not guarantee delivery over the next 24 hours due to the unofficial action at sorting offices.
Planned industrial action is also due to begin on Thursday morning.
The unofficial strike began at the Glasgow Mail Centre in Springburn on Tuesday before spreading throughout Greater Glasgow.
It then spread to Edinburgh, where about 100 postal workers called an unofficial strike at the city's main sorting office at Sighthill on Wednesday.
They claimed four workers had been suspended for refusing to handle mail transferred from the strike-affected Glasgow sorting office.
Later, Royal Mail confirmed that a "small proportion" of workers at the Aberdeen Mail Centre had also walked out.
Talks between the Communication Workers Union (CWU) and Royal Mail management on Wednesday failed to find a solution.
Royal Mail said it had only been able to make key deliveries in some areas.
Willy Marshall, branch secretary of Central Scotland CWU, said anger had "boiled over" into the unofficial dispute.
"Our workers were asked to sort scab mail, which is mail the managers should handle if our workers are on strike, and they refused so they were suspended," he said.
"We want our men that were sent home to be brought back and start getting paid again and unless that happens the strike will continue."
A spokeswoman for Royal Mail condemned the action and urged the striking workers to return to recognised negotiating procedures.
Julie Morrison, head of external relations at the Royal Mail Group, said: "Royal Mail's utmost concern is to continue to provide a service to our customers and this kind of unlawful action plays directly into the hands of our competitors.
"We hope that the union will encourage staff back to work and allow this matter to be handled following proper agreed procedures. We remain open to dialogue in this matter."
The wildcat strike has been called against the backdrop of a Scotland-wide dispute over pay, pensions and future industry changes that has seen the CWU call a series of official 24-hour strikes.
A further 24-hour official strike by delivery staff is expected across the UK on Thursday.