What to do when my mother with FTD goes through neighbor's mailbox?

Most boxes that I've seen on mailbox stands will take a small padlock. If you tell the neighbors, they may chose to use one. At least you will have done when you can.

Checking their mail reminds me of the old ‘party lines’ for landlines in the country, where you could pick up your own telephone and listen in to the neighbors’ gossip.

Do you stay home all day with your mother to care for her 24/7? If not, then she needs to either have a hired caregiver with her or be placed in a Memory Care AL or SNF where she'll get that 24/7 supervision and care she requires. I'm afraid the mail is the least of everyone's concerns with an elder who has FT dementia is left to her own devices. She could easily wander away and get lost, burn the house down by leaving the stove on after forgetting she turned the burner on, asphyxiate herself by mixing chemicals together to clean, and many other disastrous things too numerous to mention.

Isthisrealyreal has a very good suggestion for you regarding the mail situation.

I suggest you read this 33 page booklet (which is a free download) which has THE best information ever about managing dementia and what to expect with an elder who's been diagnosed with it. This booklet does not address FT dementia, which is even trickier than the other dementias and Alzheimer's and even less understood by doctors, unfortunately.

Understanding the Dementia Experience, by Jennifer Ghent-Fuller 
https://www.smashwords.com/books/view/210580

The booklet does have some good tips and tricks to help you deal with your mom and her condition, along with some Do's and Don'ts regarding her care & language needs.

Good luck.

Satoshi, welcome to the forum.

I would go knock on the neighbors door and explain that she has dementia and she is doing this, most people understand, it's the no communication that bites you with this.

Ensure them that you will return any mail she might get a hold of and be prompt in doing so.

What a challenge.

It sounds as though your mother may be seriously in need of more supervision than you are currently attempting to provide.

”People with dementia just don’t get it”, also CAN’T “get it”, and never will “get it”.

If neighbors are concerned about their mail, they really are entitled to complain. Are you planning for your mother’s inevitable and maybe soon need for someone to stay with her, to prevent this kind of problem?