Post sent to addresses in the G31 to G40 area – which covers parts of the east of the city including Bridgeton and Dennistoun – went missing for months and is only starting to get through now.
The delayed post is being delivered along with a covering letter of apology from Royal Mail, which explains a former employee is thought to be responsible for the delay, although it is not clear exactly what happened to the post in the meantime.
The letter, from Royal Mail's delivery office manager John McKale, apologised to customers for late delivery and the condition of some of the post.
The letter reads: "I am sorry the enclosed mail has been found in this condition after being allegedly withheld from delivery by a former Royal Mail employee in the Glasgow G40 area.
"I can assure you that Royal Mail takes these matters seriously and has a zero tolerance policy towards the mistreatment of mail in our care."
It is a crime to tamper with, destroy or dump mail in Scotland.
One customer who lives in Dennistoun and received the apology letter said: "I got a letter from Royal Mail attached to a piece of post.
"It was apologising for the post being so late and for its condition.
"The post was from my work and was sent in May, but only delivered this week."
A Royal Mail spokeswoman said: "Royal Mail can confirm an employee in Glasgow was suspended and has resigned after our investigation into intentional delay of mail.
"The delayed mail is now being delivered to customers in G31-40 post code area with a covering letter explaining the delay and telling them who to contact should they require further information.
"Royal Mail has a zero tolerance approach to any dishonesty and that stance is shared by the overwhelming majority of postmen and women, who are honest and hardworking and who do all they can to protect the mail and deliver it safely.
"It is a criminal offence to tamper with or delay the delivery of mail and anyone caught doing so will face serious disciplinary measures.
"This case will be handed over to the Procurator Fiscal."