There are usually two main reasons for this:
1 That you ( or somone) has set up a "rule" within your Yahoo to send certain messages to your Trash folder.
2 That Yahoo's spam filters have determined that some of your emails emanate from spammers, these auto spam filters are very good but will often, or sometimes identify false positives, it's possible to "Train" most spam filters to allow your legitimate emails.
Spam filters are often quite hard on classifying emails sent to multiple recipients or with certain key words included, as spam.
So if it's a result of 1 above then you need to remove or modify the rule.
If it's as a result of 2 then you will need to find the controls for the yahoo spam filters.
Sorry I can't help with the location for these as I have never used Yahoo mail