The union said five other workers were also suspended from Prenton Delivery Office in Wirral in July in the same dispute but some have been reinstated.
According to the BBC, Wirral residents said their post had been delayed and they had missed medical appointment reminders and wedding invitations since July due to delays created by the suspensions.
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Allan Brame, Liberal Democrat councillor for Oxton, set up a petition against the Royal Mail’s decision.
In a statement, he said: “Postal deliveries in Oxton and Prenton have been totally chaotic since the management at the Prenton delivery office suspended 11 posties.
“It seems their 'offence' was to park up at the Caernarvon Castle for a coffee during their break time.
“The reaction of the management was completely disproportionate to the perceived offence and has left people waiting for urgent mail and missing medical appointments.”
A Royal Mail spokesperson said: "As a responsible employer we always investigate any incidents where the behaviour of our employees is alleged to have fallen below the standards expected.
"Any decision to dismiss an employee is only done so in line with the clear rules and regulations that govern employment law for all UK businesses and our internal standards."
The dispute follows Royal Mail settling a 14-month worker dispute over pay and conditions, including 18 strike dates. The deal won the support of 67% of the affected union members last month (July).
Staff will get a 10% pay rise over three years and a one-off lump sum of £500.
However, the decision has been followed by disagreements over Royal Mail’s application of the sick absence policy and the £500 bonus.
At a CWU event in Leeds following the ballot, general secretary Dave Ward told union members to persist in their effort to shape the company.
He said: “As a trade unionist, you can go through the hardest battles of your life, but what makes you stay the course is that you have values different to the employers sitting opposite.
“They can throw whatever they want, but we never lose our values – the values of solidarity.”
