If there is doubt as to whether the supplier of the material is the copyright owner (for example, where a cutting from a newspaper is supplied for republication in the newsletter), ask further questions about where the material came from, so you (or the supplier of the material) can get permission if necessary.
Acknowledging the source of the work is not a substitute for permission. If you need permission to use a quote or extract, identifying the work is not enough.
If you get permission to use a work or a substantial part of a work, you will still be obliged to respect the moral rights of the author by providing sufficient attribution of their authorship.