If you decide to deny an applicant’s rental application due to information you receive from a consumer report (also known as a credit, criminal or eviction report), you are legally obligated to provide notice / letter (an adverse action notice) explaining why. If you don't, you are in danger of violating laws regarding the use of credit reports, including the Fair Credit Reporting Act.
It’s best to provide the letter in writing, but whether you send it electronically via email or mail a physical copy, make sure to keep a dated copy as a record.