Heres a few tips to ensure you successfully upload your .CSV to Mail Merge.
1. Ensure Excel is closed prior to upload.
2. There are a few different .CSV file types. You must save your file as the following:
CSV (Comma delimited) (*.csv).
3. Ensure your .CSV file does not contain any duplicate Email addresses - You can check this by highlighting your Email Column Click Conditional Formatting -> Highlight Cell Rules -> Duplicate Values.
4. Ensure all Emails are in the correct format, ie. has a .com / .org etc and the end.
5. Ensure there are no spaces after Column headings / or Email addresses.