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Email is typically very quick, but there are several reasons that email can be legitimately be delayed, for hours or perhaps even days.

1. How long does e-mail delivery typically take? What are the most common ranges?

2. How long does it actually take (more or less) for the mailer-daemon at my e-mail host (or whoever) to find the addressee mailbox is full, the addressee is unknown or otherwise undeliverable? (such as address misspelled)

3. Where the addressee data seems valid, how long will the mailer-daemon at my e-mail host (or whoever) keep trying?

Answers to these questions might be of general interest to people frequently replying and to pen pals.

I'm guessing it's really only the first that most people will be interested in, but I'll hit the other two as well.

People have high expectations of email. And most of the time those expectations are actually met.

However, what's considered "allowable behavior" will surprise you.

Computers are fast, and the internet is fast, so it's no real surprise that most of the time email is fast. It's not uncommon for email to be delivered and ready for a recipient to read or download in mere seconds of the sender having hit "Send".

A few minutes is probably much more common.

A few hours is quite possible.

A few days is also possible, and in fact, from the mail transport point of view, quite acceptable behavior.

So there you go: your email could take seconds or it could take days to get from you to your recipient, or something in between. Fortunately, most of the time delivery is much closer to seconds than it is to days, and as a result we're typically fairly happy with the rate at which email gets from point A to point B.

"... your email could take seconds or it could take days to get from you to your recipient, or something in between."

So, what might tip the scales? What might cause an email to take hours or days to reach a recipient?

  • SPAM (#1): there are spam prevention mechanisms in place that will cause email delays. For example, the first time someone emails me directly my mail server says, in effect, "I'm too busy for you now, come back later". Many spambots will not. Legitimate mailers will wait "a while" and try again, at which point the email is accepted. The "a while" varies quite a bit based on the sending server; I've seen it be a few minutes, and I've seen it be a few hours. (This technique, which is implemented on mail servers, is called "greylisting" for those interested.)

  • SPAM (#2): I've seen mail servers brought to their knees by a sudden incoming flood of spam. The server itself simply can't keep up, and legitimate email being sent to or through it can be delayed. As we know, the spam problem is huge, and I have to believe that this is a fairly common occurrence - I see it often with the big services and with mailing list providers. Fortunately, the email protocols allow for delays and retries and the like, so email typically makes it through eventually, but with an added delay.

  • Failures: Machines fail. Networks fail. Then they get repaired. As I mentioned in the previous point, the mail protocol is extremely robust and tolerant of failure. Mail servers are designed to, if at all possible, note that an error is "temporary", and as a result will keep trying for "a while" in case the error gets resolved. In this case "a while" is typically something like 5 days. You may occasionally have seen a bounce message that says "I've tried for 5 days and couldn't get through, I'm giving up."

  • Load: Somewhat like SPAM #2 above, sometimes mail servers are simply overloaded with legitimate email as well. This happens from time to time particularly with mailing list services.

  • User error or behavior: I have to include this for completeness. Occasionally, we might compose a message offline and then not connect for a day or two, at which point it gets sent. That looks like a delay. Similarly if someone only checks their mail every three days, then you might see what looks like a three day delay.

So there's lots of reasons that mail might be delayed. The good news is that if it can be delivered it typically will, eventually. The even better news is that "most of the time" it's pretty quick.

You just might not be able to count on it. Smile

As to your other, more technical questions:the mailer-daemon (mail server software) is typically processing mail extremely quickly, so decisions on the validity of email, the state of a mailbox or what have you - anything that can simply be determined by it looking - is extremely fast. Probably measured in milliseconds internally, but would appear as seconds to the naked eye.

How long a mail host will keep trying to deliver an otherwise legitimate email depends on the specific problem that's preventing delivery, and in all honesty, the decisions made by the author of the server software and the administrator of the service. Typically, failure will either be immediate, or if retrying is called for, then it will try for several days.

Sadly, and again due to spam, failure - be it immediate or after several days - may, or may not, generate a bounce or error message in return. In other words, it's frequently the case that errors happen and you'll never know.

Article C3501 - September 14, 2008 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

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10 Comments
Joop St Kloos
September 16, 2008 10:51 AM

Gratefully, you've answered a burning question.

g taylor
September 16, 2008 10:56 AM

I enjoy your e-mails.

I sent an e-mail to a friend - he said he did not get it.

A couple of weeks later I resent it.
The friend said he got my new message and then a few hours later got the original message.
Just like you said - It CAN take some time for e-mail to be delivered.

David Murphy Guajardo aka ZoftWero
September 17, 2008 12:27 AM

First of all, this is my first time writing on Ask-Leo... (i'm pretty excited XD)

A question and a comment:

1.- Is more common for an email to be late due to the crossing from one free e-mail service to another, i mean, is normal or more common, for an e-mail from a GMail user to a Hotmail user to be late or considered as Spam, than if it comes from a user of the same free service?

I ask this because i've heard lots of people saying things like "Hotmail don't like ro receive mails from GMail accounts" or "This two companies have issues..." And i would like to hear some more technical opinion...

2.- I've Used free Hotmail for more than 10 years, Yahoo! for like 5 years and GMail since it started, E-mail is sometimes a little late, but it's way better than telephone or SMS services in my country. And it can be free.

I know people like to think that the email providers are somehow intentionally blocking or delaying each other, but in my experience that's not the case. Blocks and delays happen most often due to attempts to stop spam, or simple failures, and are not targeted at specific providers.

-Leo

dunstergirl
September 19, 2008 11:43 PM

I recently had a very important (time-sensitive) message go astray (to me from someone else), and from viewing headers when it finally arrived it seems it lingered in my email server's queue for several days before being delivered even though other messages got through. A mystery, but it happens. And these were not "public" email servers in the sense of hotmail etc.

The moral is, if you are dealing with time-sensitive or otherwise important emails, either request a "read receipt" or specifically ask in the message that receipt be acknowledged, otherwise (short of a reply) you will never know if it went through or not.

And, if you are on the receiving end, be courteous and respond to the "read receipt" request (if it is legitimate mail) or reply so the sender knows it got to you.

Occasionally things are lost forever, I have had this happen as well...

I'd advise not relying on "read receipt". It's notoriously ineffective. Some mail programs don't support it, and most users ignore it even when valid.

-Leo

Leigh
November 18, 2008 4:40 PM

Hi Leo, Great website! I'm having an issue with Hotmail. The recipients on my contact list do not always receive my e-mails. These are people I regularly e-mail so my e-mail address is not on their blocked sender list. Today, for example, I e-mailed one person, they e-mailed me back, and then I replied to their e-mail and it never reached the recipient. I then forwarded my previous e-mail I had sent them from my "sent" messages folder and even copied myself on it, and neither of us received it. Any thoughts or suggestions would be appreciated. Since I'm having problems with Hotmail, I've entered my Yahoo e-mail address. Thank you!

Eric
January 2, 2011 9:58 AM

I understand that there are occasional delays, but my niece recently received an e-mail from me dated from 4 years ago. How is that possible?

Wow. No idea. Lots of wild speculation (crashed server restored from a 4 year old backup? Random things like that), but nothing really sensible. I'd look at the headers and see if possible where the delay was: Why is some email to me delayed by days?
Leo
03-Jan-2011

Ted
May 1, 2011 9:45 AM

Does it take any longer for an e-mail to be delivered from one country to another?

Not usually, no.
Leo
01-May-2011

Mark J
May 1, 2011 11:05 AM

Ted: International email does not usually take significantly more time , but in some cases it can take longer.

Glenn P.
September 20, 2011 10:24 AM

Eric asked:

"My niece recently received an e-mail from me dated from 4 years ago. How is that possible?"
I don't suppose you were using <http://www.FollowUpThen.COM> ???

Rod
September 27, 2011 6:20 AM

As a small business, we recently signed up for Google GMail for business. While it costs $50 a year per account, it sends and receives immediately (and internationally), is really stable and has great spam capture.
Note: Check your Spam folder occasionally to make sure it has not captured an email that is not spam!
One more thing: GMail uploads your attachments before you send the email, so when you hit "Send", it goes immediately.

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