The Queen has been given a rousing rendition of Happy Birthday by Royal Mail staff - and jokingly suggested that she may have added to their post bags.
On the eve of her 90th birthday, the Queen toured her local Windsor postal depot, which was renamed in her honour, accompanied by the Duke of Edinburgh.
And as she met staff at the newly named Queen Elizabeth delivery office, the monarch suggested to Royal Mail Group chief executive Moya Greene that cards and gifts from well-wishers may have added to the huge number of items they deliver every year.
Thursday's birthday is one of many milestones in her record-breaking reign. She is already Britain's longest reigning monarch but on April 21 she becomes the country's first nonagenarian sovereign.
There was a party atmosphere outside the depot, with long-serving staff from across the country invited along with their family and friends, and all waved Union flags.
Two choirs made up of Royal Mail staff from London and another group of singers from Bristol sang Happy Birthday to the Queen as she left in bright spring sunshine.
The Queen's visit on her last day as an 89-year-old saw her commemorate the anniversary of another British institution - the postal service.
Five hundred years ago, in 1516, Henry VIII knighted Brian Tuke, the first Master of the Posts - a move that was the catalyst for the creation of the Royal Mail we know today.
Jan Warburton, 60, who delivers post to staff living at Windsor Castle, was introduced to the Queen.
She said: "When Moya said we deliver over a billion parcels a year, the Queen said 'I've probably added to that this week'."
Ms Greene gave a speech before the Queen unveiled a plaque to mark her visit to the delivery office and paid tribute to the monarch.
She said: "Your Majesty, the leadership and commitment to public service you have shown throughout your lifetime continues to inspire us all.
"It is our privilege to carry your cypher on our iconic post boxes and your image on our postage stamps, each of which receives your personal approval before they are issued.
"Ma'am, we are enormously grateful to you and the Duke of Edinburgh for gracing us with your visit today on the eve of a significant milestone of your own.
"It is typical of you both to put service before self in recognition of this unique and special day in Royal Mail's history. I am delighted to announce that from today the Windsor delivery office will be known as the Queen Elizabeth delivery office.
"On behalf of all of Royal Mail's employees across the UK and here in Windsor, may I wish you a very happy birthday for tomorrow."
Ms Greene added to laughter from the gathered staff and senior managers: "I have it on good authority that your own postmen and women will be especially busy with tomorrow's mailbag."
During her visit to the delivery office, the Queen also met postal workers Frank Zecca, 55, from Ascot, and Tracey Stacey, 50, from Old Windsor, who staff the parcel collection counter.
The Queen and Philip were shown a small handheld electronic device called a personal digital assistant which allows staff to scan the barcodes of special delivery or tracked items and check their whereabouts.
Ms Stacey was moved to tears by the visit, saying: "It was very emotional experience. I don't know ... it was just the build-up of it all because it's the Queen obviously.
"She's just a person like us, she does all the things that we do, but she's the head of the country - I think a lot of people aspire to meet her."
She added with a laugh: "But it was five minutes of sheer panic".
Ms Warburton, who takes mail to the Queen's Berkshire home, said the post for Windsor Castle is sent to the delivery office in the Berkshire town but the "Queen's mail and other post" goes to London to be checked and is delivered by a royal postman while she delivers mail to staff at the castle.
She added: "It's just like an ordinary delivery really - but, yes, it's special. Who wouldn't love to live in a castle?"
The Queen was shown a storyboard chronicling the Royal Mail's history and a display of stamps with a royal theme from the past 50 years.
And with her birthday just around the corner, the Queen received a framed edition of the stamps produced to mark her birthday showing her , the Prince of Wales, the Duke of Cambridge and Prince George.
She was also given a sterling silver hallmarked fountain pen and Philip made the postal staff laugh with the quip: "Can you afford it?"
After Happy Birthday was sung by the two choirs, the Queen received bouquets of flowers from 18-month-old Charlie Capper - believed to one of the Queen's youngest posy boys - who was in the arms of his grandfather, Derek Warner.
Mr Warner, 69, from Horley, who celebrates 50 years as a Royal Mail employee in May, joked about his grandson, saying: "All the time he wanted to get into the Queen's Bentley we had to turn him around but when he handed the flowers over, he did it just right."