What is a pre-send check?
A pre-send check reads a campaign step together with the leads it is about to go to, the way the merge will, before anything is sent. It catches the mistakes that only show once the two meet: a merge tag with no matching column, blank cells, broken spintax, the same lead twice and addresses that cannot be sent to.
Reading the result
Each merge tag is matched to a column, exactly or ignoring case, spaces, _ and -, so first_name finds First Name, and the match is shown. Gaps are counted over the rows that would be sent: valid addresses, each once. The send estimate uses only your numbers: leads times steps, divided by inboxes times daily sends per inbox, rounded up to whole sending days. Nothing is uploaded.
Common problems it finds
A tag your file does not have
{{company}} in the copy and Company Name in the file do not match, so the tag prints nothing for every lead. Rename the column or the tag.
Blank cells behind a tag
One empty first_name in a thousand rows is one email that opens "Hi ,". The checker lists the rows, or you can add a default to the tag.
Duplicates and invalid addresses
A repeated address gets the step twice, and a malformed one bounces or is skipped. Both are counted, with examples, and left out of the gap counts and the estimate.
A schedule that does not fit
The estimate counts sending capacity only. Each lead waits between steps, so the last leads finish later than the estimate. It never assumes a sending limit: the numbers are yours.