While it is possible to use an external vendor for a mass email, CU and CUIMC spam filters may score these as junk, causing them to end up in the recipient's junk folder.  To prevent possible issues:

If you are communicating to a fairly static list of people in your department or otherwise internal to CUIMC please consider using a Microsoft 365 Group, CUIMC internal mailing list, or a Contact Group in Outlook.

Some issues may be prevented by making sure the mass mailing from an external vendor uses @cumc.columbia.edu email addresses for CUIMC affiliates, rather than their "virtual" @columbia.edu address.

Work with your Certified IT Group to request whitelisting for the vendor before sending out an email. Technical information about the sending address can be marked to bypass the spam filter.