Skip to content
Thoughtful, detailed coverage of everything Apple for 34 years
and the TidBITS Content Network for Apple professionals

Question: Are addresses checked when sending mail?

Question: Are addresses checked when sending mail? Kurt M. Scudder <[email protected]> writes from Denmark wondering if mail programs – Eudora specifically – contact the recipient’s computer when sending outgoing mail. "I can see this in the progress window. In fact, on some occasions, Eudora will freeze there, returning a message some minutes later saying that it couldn’t contact the recipient. I thought that when I sent mail, it simply went to my SMTP host, and I didn’t have to worry about it after that."

Answer: Kurt’s right. Most Internet email programs send messages via an SMTP (Simple Mail Transfer Protocol) server that then connects with remote mail systems to deliver mail. The SMTP server queues mail – that is, receives it, puts it in line to go out, and then sends it.

The problem Kurt is noticing is that most SMTP mailers verify two things. First, they check to see that the full domain name of the recipient – the entire part following the @ in an email address – has a corresponding mail server. Second, they look to see if the username exists when it’s an email address for someone that has a mailbox on the same mail server. Some servers do this "live" – as the mail is being sent from your mail program – and others – like Qualcomm’s Eudora Internet Mail Server (EIMS) – do it after receiving the mail from you.

So, let’s say you send mail to <[email protected]>. Your mail program connects to the SMTP server that you specified in your preferences and tells it the destination address. That SMTP server then looks up whether a mail server is listed for using DNS. (DNS or domain naming system is a distributed method of pairing a name – like – with an Internet address or mail server.) If the SMTP server finds an appropriate mail server record for, it then accepts the mail from you and sticks it in a queue because it now knows that it can send mail to

So the delay Kurt sees is when either the DNS server that holds the information for that particular domain is down or slow – or the connections over the Internet between Kurt’s ISP and the remote system are sluggish. The standard Unix SMTP server, sendmail, won’t accept email when it can’t find the destination machine, while EIMS will accept it and later send an error message back to you.

The second problem described above occurs when you’re sending mail to another user whose mailbox is on the same server. If, for instance, you mis-type the user’s name or the mailbox has been deleted, you’ll get an instant error telling you that no such user exists. [GF]

Subscribe today so you don’t miss any TidBITS articles!

Every week you’ll get tech tips, in-depth reviews, and insightful news analysis for discerning Apple users. For over 33 years, we’ve published professional, member-supported tech journalism that makes you smarter.

Registration confirmation will be emailed to you.

This site is protected by reCAPTCHA. The Google Privacy Policy and Terms of Service apply.