Why some mails sent through asp.net web application goes in spam?

[Napsterr]

I have a asp.net web application through which I am sending mail to my website users.
Sometimes the mails i sent goes in spam for some users.
Most of the times this thing happens in case of yahoo or rediffmail users.

Is this problem with the .NET or this happens due to any other issue?

Please help me regarding this.....

Thanks in advance.

[Plater]

Property of their spam blocking.
Look up commonly triggered on items for spam filters.
*Things like having an ip address in the email.
*Using any commonly blocked words, i.e. things related to medicine or sexualy explicit terms.
*Sending to too many recipients in one email
*etc

[Napsterr]

Thank you Plater, for reply
In my case, I don't have
1. IP address in the email
2. Things related to medicine or sexualy explicit terms
Only the thing is that, I send multiple mails at a time, but in my To section of mail there is only one email address at a time.
Still it goes in spam, Why?
Can it be avoided anyhow?

Thanks