Out of all the online tools I've tried and tested, I can't believe I haven't talked about this one, one I've used over 4 years at this point and continues to add features that help. It's how I found many emails to politely and persistently ask about my application while sending relevant links to this blog but there's a lot more down there too! I uploaded the .CSV of emails to recruiters I use to send personalized, yet still batch, emails through Google Sheets to check for validity and acceptance rates. It did work, but I retracted everyone's information. I had used the tool to check a few individual addresses and it did find one that was no longer operational; Suppose the person moved onto a new role. I also converted it to a Campaign to send mass emails - which does have a neat thing to check how spammy one's email may be, though it doesn't give hints on how to better coerce click through and action for recipients. It seems to correlate doing...
I research and implement products + infrastructure for the Cloud, software, and other interesting things. My tailored Cloud Project Portfolio is at runtcpip.com