Suddenly Emails Bouncing 5230

What else does it say along with just the number? Is there something about the sending IP address being blacklisted?

If you're trying to send email directly, using simple SMTP from your Linux machine to the destination domain, one possibility is blacklisting of blocks of IP addresses. Here's the history (at least the part I'm aware of):

In the early days of the internet (apranet, etc.), SMTP was designed as a way for any machine to send email to any other machine. This worked very well for a long time and arguably still does, where it is allowed to continue.

Then, spam happened. Among other methods, spammers started using infecting machines running TOOS with the worm-virus of the week, turning those machines into zombies that would send large amounts of spam in behalf of the spammers. Other worms-viruses-virii of the week send large amounts of email to replicate themselves on vulnerable TOOS hosts.

Then, certain misguided, snobbish souls decided they would refuse to accept direct email from any machines other than certain "blessed" machines, mostly ISP outbound-outgoing mail servers. In particular, they make lists of blocks of IP addresses used for dynamic IP DSL and refuse to accept direct email from IP addresses in any of those blocks. Often, the makers of the lists claim innocence, because they just make the lists but don't directly refuse any email delivery. (They only advertise their over-broad lists as ways to reduce spam.)

Given that the blocklisters-blacklisters-snobs are firm in their misguided convictions that the way to beat spam is to prejudicially refuse direct email except from certain "blessed" hosts, the solution is you have to send through a "blessed" host. For a while, I used a Perl script I called "sendsnob" that would send email through my ISP's "outgoing" server. Later, when the number of snobs grew, I reconfigured postfix to use the transport table to route messages intended for certain destinations through the ISP's "blessed" outgoing server, using what I call a snob list. Another option is to configure your mail client (mutt can do this) to send through your ISP's "blessed" outgoing server.

Good luck.

Robert Riches (Yes, that is one of my email addresses.)