Sometimes you may try to guess someone’s email address - you may know their name and their domain, and have examples of how their company structures their employees’ email addresses, e.g. firstname.lastname@companywebaddress.
There is no 100% sure-fire way of doing this without actually sending an email yourself, and seeing if it works - actually, there is - you can pick up the phone and ask the person you want to send the email too for their email address, a novel idea but it should work.
verify-email.org is one website where you can try to determine whether the address you think might work, has a good chance of finding its way past the company mail server and into someone’s inbox. Simply type the email address into the box and click the verify button.
This service confirms, first of all, whether the company domain name exists then it will check if the server will let an email addressed to a particular person through. It does this by sending the same commands to the server which I real email would do, and seeing what the response is from the server. If the address is no good, then the server will send a response back saying so.
You can achieve the same if you know how to use an oft-ignored service which lurks on many computers called Telnet. Telnet was developed way back in 1969 but it’s still useful today in certain circumstances - Telnet commands are actually used by the above service.
Please remember that this system is not foolproof - most mail servers are configured to lie in an attempt to stop spam mail from getting through, and the first line of defence is to either say that all addresses exist (whether they do or not) so the person or robot checking doesn’t actually know if they have a valid address or not (a valid address is a saleable commodity) or, in some cases, to say that all addresses don’t exist the first time someone tries to send an email to a particular address (this is known as “grey listing”).