Here is guidelines for you can setting up Folder Auto Clean in SmarterMail

The folder auto-clean feature in SmarterMail is designed to help system administrators keeping mailbox size(s) under control. Using folder auto-clean, common folders like the Junk E-mail, Sent Items and Deleted items can be regularly purged of old messages  

1. To create rule for the Folder Auto Clean, user could log in to the webmail,

2. Click at Setting > Filtering > Folder Auto Clean

3. Click on "Override auto-clean settings for this account".

4. Click on "Rules" >> Add Rules

5. The windows below will pop up for you to configure the Folder Auto-Clean, you may insert your preferred setting, You may choose based on email folder size/ date.

Referring to above sample setting, it will remove the emails which mail age is more than 30 days.

Note : Please take precaution on setting up Folder Auto-Clean as email deleted is not retrievable.