OK, since you indicated the contacts are in your public folders, then there must be an Exchange server involved. In that case, 1. open you public folders and click the folder with the contacts. 2. Open a new message and click the “to” button. 3. Under the heading “address book” at the top you will see Global Address List with a drop-down arrow next to it. 4. Click the drop-down and under “Outlook address book” you should see the name of that public folder. If you do not, you probably do not have that contacts folder highlighted in public folders. Close everything out and go back to step one. 5. If you see it, click that and you will get a list of all the contacts in that folder. 6. Highlight all the contacts on the list by clicking the top one, holding the left shift button, scrolling to the bottom and clicking the bottom contact. 7. Then click the “to” button at the bottom and all will be entered into the message “to” field.
You could create a distribution list inside the public folder contacts depending on how many are in there. There is a limit to how large the Distribution list can be.You could create multiple smaller distribution groups to work around that.