In moving to Constant Contact from a previous email marketing platform, I was trying to Add Contacts From File using a 2MB .csv with 6,225 contacts. Only 1,670 were actually being added with 1 error (pretty far off from 6K). Re-saving my contact list as a .xlsx file type and re-uploading provided a workaround for the issue. I wanted to post this up here to hopefully help other Constant Contact users experiencing the same problem and to bring the issue to the attention of the Constant Contact Support Team.
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