Issue Finding last row to send mass emails

Hi, I'm new to VBA coding and having trouble with the last aspect of my code to send mass emails to everyone in an excel database. The Code operates correctly as its sending emails when I specify which row to loop until. Though, When I try to change the loop until statement to loop until it finds an empty cell it keeps returning an error that reads: "Outlook does not recognize one or more names". Not sure why Outlook is recognizing any names since everything is specified through excel.

Here's the code I'm using:

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Any errors you can identify would be greatly appreciated.

Thank-you.