Generally, relaying has a very specific definition, but I’m not certain it applies in your case. Your email provider might be using it in a non-standard way.
Either way, relaying is a fundamental way that email travels from your outbox to your recipient’s inbox.
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Email from me to you
Conceptually, email is very, very simple. I address a message on my computer or device and send it to you. You receive it on whatever device you use for email.
My mail server (askleo.com in this case) sends it to your mail server (randomisp.com in the example above), from which you can retrieve it.
While it wasn’t always so, I’d venture to say that well over 95% of email is sent this directly: sender’s mail server to recipient’s mail server.
Mail relay: example
Email need not be routed directly. It’s possible for an intermediary server to be involved.
In this example, rather than sending the mail directly to you, the originating server (askleo.com) delivers it to a third-party intermediate server (“somerandomservice.com”). Once the delivery is complete, that middleman server delivers the message to your mail server.
That middle server has relayed the message by passing it on to its destination. There’s no limit to the number of intermediate servers that could be used. My email to you could be relayed through dozens of servers, if need be.
It’s rarely needed today, but when the email system was originally developed, the concept of relaying was required because there was no way to guarantee that a server like ask leo.com could connect directly to randomisp.com.
These days, relaying is often a side effect of how sites and servers are deployed. For example, several servers might share a common intermediary server for email delivery through which all outgoing email is routed.1
Mail relay: definition
If a server receives an email for a domain that it hosts — such as my askleo.com server receiving email for firstname.lastname@example.org — it need do nothing other than deliver that email to my inbox.
On the other hand, if a server receives an email for an email address on a domain (like randomisp.com) that does not reside on that server, it has two choices:
- Bounce or ignore the email: “I don’t handle email for randomisp.com.”
- Pass the message along to the real randomisp.com server.
Servers that make the second choice are called relay servers.
The open relay: a spammer’s dream
Let’s say you’re a spammer and you want to send email to lots and lots of people. You can:
- Send that email directly to each email address’s server. You’ll quickly be identified as a source of spam and blocked.
- Sent that email to a relay server that accepts incoming email from anyone and send it to anyone — what’s called an open relay.
As long as that open relay fails to block you, you can send as much email as you like to anyone you like.
Needless to say, open relays are a bad thing. The correct configuration insists that either of two conditions be met:
- The incoming email is destined for an email address on the server. So, for example, the askleo.com email server will accept email from anyone as long as the email is being sent To: an askleo.com email address.
- The server asking to relay email must itself be authorized to do so. This is usually done by having some kind of email administration account the server uses to log in to the relay server.
In effect, a well-configured mail server will say, “If the email’s not for someone on this server, and I have no idea who you are, you can’t send. Go away.”
Technically a relay, but not usually called one
If we look a little closer at the simple case we started with, we see it’s a little more complicated than we originally envisioned. If I send you an email:
My PC, running a desktop email program like Microsoft Office Outlook or Thunderbird (not a webmail interface in your browser), sends the email to the askleo.com email server, which sends it to the destination mail server, randomisp.com. The next time you load your desktop email program, the message is copied or downloaded to your PC.
In a sense, each email server acts as a type of relay. In the first step, askleo.com relays my message between my PC and your email service, and in the second step, randomisp.com’s email service is relaying incoming email from askeo.com to your PC.
Technically, we don’t refer to this as relaying, even though it kind of is. The term generally refers to email-server-to-email-server relaying, without consideration of the PCs at either end.
However, this looser interpretation of relaying is the only explanation I can come up with for your email provider’s choice of words: that the “account had exceeded the relay quota for the day”. That such quotas might exist is nothing new — it’s one more thing we can blame on spammers having made necessary, for exactly the reasons your friend encountered. Rate limits or quotas are one way that ISPs limit the damage — i.e. spam — from such a situation. What’s odd is the choice of terms. Normally, it’s simply referred to as a sending quota, not a relay quota.