The woman was staying in a mother-baby care unit for treatment for her depression when she had an encounter in a café that reminded her "the smallest kindness can make a difference".
The note read: "Dear café owner. You must have thought I was a bit strange when I stood outside your cafe trying to open the door when you had clearly closed."
It continued: "You might not remember me, I had a baby in a pram and a hospital wrist band on. I wanted to thank you for inviting me in out of the heat and kindly making me a coffee, even though you probably wanted to go home.
"When you asked if I'd walked over from the hospital, I was too embarrassed to tell you I've got post-natal depression and my baby doesn't sleep and I'm just exhausted.
"Walking to a café for a break was what I needed as much as the coffee.
"Your kindness made a difference to my day and reminded me of the saying: 'Be kind to everyone you meet because you don't know the battles they're fighting'. Thank you."
The post, which was uploaded on PANDA on 24 November, has prompted other mothers suffering from postnatal depression to share their own stories.
"Cafés were my saviour when I had PND, a reason to get out and feel part of life and not just invisible at home," wrote one mum.
"It was therapy to me and I now take my five-year-old to a café whenever we feel down. They are wonderful places."
Another mother commented: "The smallest acts of kindness mean the world when you're battling depression, it really is like a bolt of sunshine through the clouds.
"I remember being extra-sensitive to acts like this and feeling the need to express my gratitude so that the person knew they had made a difference. Bravo to this café owner."