To save sent mail in IMP:
1) Login to IMP webmail.
2) Click Folders and create a folder to store your mail or you may use an already existing one.
3) Click Options – then click “Personal Information” under the Your Information column.
4) Click “Edit your identities” and create an identity. When doing so CHECK THE BOX “Save sent mail” and then specify the folder to save it in from the dropdown list.
5) Click Create.
6) Select your identity and make it the default identity. Once you have done so then when sending it will save a copy in the specified folder.