Just the fact that someone used your service is a signal for their current employers, it might be used against employees during lay-off rounds as interpreting it that they are 'on the market'. In the current employment climate that is super dangerous. I strongly urge you to reconsider this re-use of data, especially for EU citizens where all use other than the one for which the data is gathered is illegal. See also: GDPR, specifity as well as the section on mandatory opt-in for future use.

Note that you are opening yourself up to major legal and financial liabilities, besides the obvious personal ramifications, ie: you're on the record as a sleaze unless you handle this with velvet gloves from here on in.

https://en.wikipedia.org/wiki/General_Data_Protection_Regula...