I shall be glad when the post office workers go back, it's very difficult not getting any letters.
Can I say it's very difficult not to get any letters?
1. It's very difficult not to get any letters. (according to bhaisahab, incorrect)
2. It's very difficult not getting any letters. (correct)
3. It's unhealthy to be fat. (correct)
4. It's unhealthy being fat. (correct)
Why is #1 incorrect?
First, I'll try to explain the problem. In many cases, you could say it either way, (as you have in 3, 4) and transposing the sentence as I did, could change the wording:
Being fat is unhealthy -> either 3 or 4.
But sometimes it makes a difference, if "<verbing> something" doesn't mean the same as "to <verb> something" - which has discussed here often.
In the case of "receiving letters/ to receive letters" there is a difference.
In fact, 1. is correct if you mean it this way: I don't want to get letters, but it's difficult to avoid it. In this sense, you might say, " "1. It's very difficult not to get any spam emails."
This doesn't mean: "1a. Getting no spam emails in very difficult". which was your meaning, ie. "I'm unhappy when I don't get spam emails".