There are many alternatives to the phrase "hereby confirm that" that can be used to effectively communicate the same message. Some of these include "certify that," "attest to," "affirm that," "verify that," "declare that," "state that," and "assure that." Other possible options are "warrant that," "guarantee that," "validate that," "ratify that," "endorse that," and "avow that." Each of these synonyms can add variation and interest to a document or email, making the message clear and professional. Choosing an appropriate alternative to "hereby confirm that" is important in conveying confidence and credibility in any type of formal communication.