Most Aussie ISPs (and most email servers for that matter) have blocked SMTP relaying, as a measure to reduce spam distribution. They will now forward emails only from direct logins to the domain.
For example, you may have email accounts with BigPond, iHug and TPG. If you are on a PC that is connected to BigPond, you can receive email from all three accounts, but you can only send via the BigPond SMTP server.
I kept trying to send emails to others at the same address and was able to get through to them, so I’m still a bit confused.
I am with bigpond.
And I still haven’t worked out how to view a blacklist in Avast! or Comoda to check if I’ve accidentally blocked them out.
Is it worthwhile uninstalling and installing the programs to see if it makes a difference?
I’m pretty sure the root cause of the problem is in how your email software is configured. Providing it is configured to ONLY send outbound email through “mail.bigpond.com” (or whatever BigPond email server you’ve been assigned to), you shouldn’t ever receive any error messages about relaying.
For exapmple, if I configure an email app to send outbound emails via mail.bigpond.com and I connect to the internet via a TPG account, any emails I send would result in a relaying error message. If I leave the email app they way it is and connect to the internet via BigPiond, I would then be able to send outbound email successfully.
All major Aussie ISPs have done this to prevent “outsiders” sending spam from their email servers.
Assuming that you have multiple email accounts, configure your email application to receive incoming emails via the respective POP (inbound) server for each account, but all accounts must be configured to use the SMTP (outbound) server of the ISP you are connecting to the internet through.