BleepingComputer.com: Thunderbird error (reason: 553 5.3.0)

sometimes when I send a response on Thunderbird I receive the following error:

reason: 553 5.3.0

But if I compose a new email to the same person it goes through.

Scratch, scratch (my head)...

Any ideas on how to cure, or what is causing this?

Thanks!

See if this applies >> Emails with a Dot in Prefix Bounce When Replied To

Some said that it was/is Thunderbird's fault & others argued that it was on the server side. No one mentions though if forwarding to the same recipient instead of replying has the same effect.

Thanks for the reply, I also thought this might be an issue with Thunderbird, but apparently not.

Here is the complete error message, with the emal address changed to protect the innocent (notice there is no dot in the address):

(reason: 553 5.3.0 <someone@arandomurl.com>... Rejected by Iserv RBL. If this is an error, please contact our customer service department at 1-888-644-7378.)

I think I will call them now.