When I try to send an email I get a 'relaying denied' error message

This is usually an authentication error and happens because you are trying to send mail using an SMTP (send mail transport protocol) server that doesn't recognise who you are.

The problem being described by the error message is the SMTP server isn't able to identify you, so will disallow the sending of email via your mail server.
This is a security measure that is put in place to prevent unauthorised people (spammers in particular) from using your account's SMTP mail server.

Most SMTP servers are 'closed' which means that only authorised users can use it to relay (send) email.
If the SMTP server was always open for unauthorised email, then absolutely anyone would be able to use it.

If someone else used your SMTP server to send out millions of 'spam' emails then:

1. It would 'burn up' your monthly bandwidth allocation.
2. You may incur an excess bandwidth charge.
3. Your SMTP server would be identified as the origin of the spam, which would result in your hosting service being cut off for breaching our terms & conditions of service.
4. You could become liable for prosecution for apparently sending out unsolicited 'spam' email.

Simple things to check if you can't send email:

If you are using your ISP's SMTP to send your email then the first thing to do is to check your outgoing mail settings...  e.g. If your dialup ISP is dialukt.co.uk then your outgoing SMTP is mail.dialukt.co.uk

Next... Check that you are actually dialling in to the same ISP who's SMTP you are trying to use!
It's amazing how many support requests we get where we discover that the user is dialling in via one ISP and trying to send their email via another ISP's mail system... This definitely will NOT work!

You can only relay email via an ISP's mailserver if you are dialled into their system.

 

UKT Customers - How to 'open' the SMTP relay to send your emails:  If you are a UK Trading customer and have a website hosting account, then you will need to collect your email from the server first, before trying to send. Our system uses pop-before-relay, so will use the POP login information as authorisation. By collecting email  you will have just logged into your server with a username/password. The server now knows that you are authorised to use the SMTP server, and will log your IP address, and will then open a relay 'window' for 15 minutes. The SMTP 'relay window' will stay open for 15 minutes for sending mail, and then it will close down again. If this happens you'll need to collect mail again, and the 'relay window' will again open for 15 minutes. Why only 15 minutes? You have to bear in mind that while this SMTP 'window' is open, in theory anyone with an 'authorised'  IP address can use it to send mail, as well as you. The chances of someone being able to find the open SMTP server and use it during this time period is fairly remote whereas, they may be able to exploit it, if it were open longer. ADSL users.... If you set your email client to collect email every 10 minutes, this will keep the SMTP server up to date with your IP address, and enable you to send your email at any time. If you have a fixed IP address with your ADSL service, then we can insert that IP address into our MX records and you will always be able to send email from your account's SMTP server. (£15 charge applicable)

A 'Rough' Technical Explanation: Send Mail Transport Protocol

Every mail server requires you to identify yourself before allowing email to be sent.

When you dialup to the internet, your dialup service checks your username & password, and logs you into their system where you are given a temporary IP address.
When you send mail using your dialup ISP's SMTP system, the SMTP server looks at your IP address and if it belongs to your dialup provider then you are allowed to send mail.

However... When you try to send mail from your domain's SMTP at UK Trading, the SMTP server has no idea who you are because it doesn't recognise your temporary IP address... for obvious reasons! Because the IP address belongs to your dialup ISP and not UK Trading.

When you log in to collect your email, your email application opens a connection to the webserver and sends it a username/password combination. If the connection is authorised (correct username/password) then your email application will download your mail.
This login is transitory and is only normally valid whilst  mail is being downloaded, then your email application closes your connection to the server.