Why some mails are sent to Spam?

Perhaps it triggers something in the Spam algorithm. Perhaps it is something in the way it is addresses.

A word in the e-mail may be in your Spam filter so it is being identified as Spam. (I first read it as “Spain” and was quite amused; bad night’s sleep.)

Spam is not the same as a virus.
Different spam detectors tag it differently.
They use sender reputation scores and many other considerations to make that determination.
There are different levels of spam detection that you can also control on the client side.
I worked in one company that turned away 4 times the inbound mail because they were considered spam. We processed 10 million messages a day and 30 million were spam.
It is a huge consideration in corporate email if you imagine how much larger the infrastructure that would be needed if all the spam was let in and stored.

It’s part of the learning process for your spam filter.
You should be able to mark any emails that you don’t want to be marked as Spam, as “Not Spam”.
Your spam filter should then eventually be able to recognize that distinction.

I don’t know, but it’s annoying.
There are two email addresses that always go to spam in my email program. They are in my address book and I’ve marked them as not spam several times…but they continue to go to the spam box. Why is that? It’s odd. One is a yahoo email address; I think the other one might be hotmail. My own email address is through my ISP.

Read the message header of the email in question and it should give you an indication.

Some email are going into spam folder. Because spam filters look at two main things when trying to determine if a message is spam or not. First, the headers, which indicate where the message is coming from, then the subject and body of the message. The content of the message is then investigated for common words, phrases, images and general structure which might indicate that it is spam.