Managers have begun delivering post after a mass walkout at a Royal Mail delivery office in Somerset.
About 100 Bridgwater staff walked out in protest at a colleague's suspension and over a dispute about time off for medical appointments, the Communication Workers Union (CWU) said.
About 64,000 items of mail need to be posted to about 34,000 addresses across Somerset, Royal Mail said.
The firm has said it is disappointed and is trying to resolve the dispute.
"It's an unofficial dispute that's going to affect mail to postcodes TA5, TA6 and TA7," the CWU's Dave Wilshire said.
Talks between the two parties are ongoing.
A spokesman for Royal Mail said: "We are working hard to resolve this unofficial dispute as soon as possible and contingencies, including managers helping out with deliveries and collections, are in place to keep the mail moving and minimise disruption to our customers.
"We are prioritising special delivery and Royal Mail tracked items and we are updating our website with the latest information."
Acknowledgements to Darrell Herd and BBC Somerset