The client wants an online email validation script. It should use the server RAM, Bandwidth of a LInux Server to clean and validate list of email addresses supplied in format of TXT, CSV, XLS, XLSX files. It should be capable of multi-threading so it can process 1000s of emails every minute or so. It should be capable of imitating a genuine SMTP connected and disconnecting before sending a test mail.
The accuracy that they are aiming to achieve if close to 93-95%. You can use any server technology and scripting language. The admin interface should contain facility to upload all the lists one by one and schedule it by dragging according to what admin considers as high priority. There is NO backend. Once the list is cleaned, it should produce 3 files - Valid Emails, Invalid Emails, Timed Out Emails.
It should function similar to Brite Verify but it doesn't need to have frontend for real-time validation but similar technology must be developed.