The SMTP server you currently have set in the Mulberry preferences has been configured to prevent you from sending email to recipients outside of its 'domain' if you connect from a computer that is also outside of its 'domain'. This is done as an anti-spam measure.
To fix this, change the SMTP server in Mulberry's preferences to the SMTP server provided by the ISP that you use to connect to the Internet.
Also, check with your site admin to see whether SMTP authentication is supported by your server. If so, then leave the server as it is, but turn on Authentication instead.
What is relaying?
Relaying is the process of submitting a message to one SMTP server, with the purpose of having it sent to another server for delivery. This will happen if the server you submit a message to is not the server that handles delivery for the recipients of the message. For example, say you send mail to 'email@example.com', and use 'smtp.user.com' as the SMTP server. That SMTP server handles delivery of mail to users in the 'user.com' domain, so it can directly handle the 'firstname.lastname@example.org' address. Instead, if you addressed the message to 'email@example.com' but submitted it to 'smtp.user.com', that SMTP server would have to pass the message to the server that handles delivery for the 'group.com' domain - the process that is called relaying. SMTP servers have tables and lookup mechanisms to enable them to talk to each other for the purposes of deliverying messages to the right person, across different domains.
Why is relaying disabled?
Relaying is frequently a tactic that spammers employ to try and disguise the origin of a spam message. They generally find an SMTP server that is open to relaying, and submit their spam messages for delivery to a whole group of people (most of which are likely not to be in the domain handled by the SMTP server).
To prevent this abuse of their SMTP server, many server admins prevent relaying for anyone who is not authorised to do so.
How is authorization for relaying determined?
In most cases, SMTP servers look at the IP address (internet address) of the machine connecting to them for submitting a message. If that IP address is not in a recognised range, then relaying will be disabled. Sites can usually configure their servers to accept relaying from computers physically located on their own local area network. If you dial-in via an ISP, the IP address of your machine will be assigned by the ISP, and won't be in the range recognised by your site's SMTP server, so relaying is denied.
Another solution for authorizing relaying, is to use the SMTP AUTH (authentication) extension. When this is available, the client machine authenticates to the SMTP server, much the same way it does to retreive messages. If authentication succeeds, then the SMTP server can enable relaying as it knows that the authenticated user is legitimate and is not, hopefully, a spammer.
How do I setup Mulberry to prevent this error from appearing?
Ths simplest solution is to change the SMTP server address to that used by the ISP that you use to connect to the Internet, when you use the ISP's connection. For laptops that move from home to work, you'll have to change the SMTP server each time you change its location.
The other option is to check with your site administrator to see whether SMTP AUTH is available and what authentocation schemes are supported. If it is supported, you can change the Authentication settings in Mulberry SMTP account panel to enable authentication, and that should allow relaying. In thiscase you can leave the SMTP server address set to your site's server, no matter where you connect from. If your site doesn't currently support SMTP AUTH, request that it does, as it makes handling the relaying problem much easier.