The message suggests to me that one of your correspondents have their email account provided by an organization which uses a "DNS Blacklist" - a list of sites or, more specifically, IP addresses reported to have been sources of malware or spam. And your ISP, or your particular IP address may be on such a blacklist.

Another possibility is that somehow your own email address has been added to such a blacklist - though I must say I haven't encountered such a service.

Or even your own ISP has backlisted you; but if they thought you were running a compromised machine, I'd expect them to to be talking to you about it.

In any of these cases, your ISP or email provider is probably the most useful place to go for help with this.

Can you send successfully from a different email address? Can you send successfully via webmail? (Email providers are generally more successful at keeping their webmail systems off blacklists; but then again, few spammers would use webmail; it's too clumsy and limiting, and it's much harder to spoof the sender.)

Thank you very much for your quick reply. Thunderbird still works with a different e-mail-address. I can send and receive messages. I can send e-mail from the affected address successfully via webmail. However, I have contacted my e-mail provider regarding the blacklisting. So, I will wait and see what they suggest. Thanks once again for your quick support.