A possible reason for this may be that in the near past a bot misused lunacleaning.ro and lunacleaning.ro temporarily refuses all requests to sent.

You can only wait, until lunacleaning.ro again accepts mails to send, or try to find a public SMTP server (fortunatelly in Thunderbird you can switch between different SMTP servers).

I assume that not youself are the bot sending thousands of identical mails?

Of course not. We have no knowledge of anything like this going out from our email. Should the public server info be provided by our hosting provider? Thank you

You have to make a contract at a "free" provider, like gmail or web.de (maybe there exists web.ro). You then can use the smtp settings of that contract to define the additional SMTP server in thunderbird.

Thanks so much! We'll have to try that.

https://tools.dnsstuff.com/#dnsReport|type=domain&&value=lunacleaning.ro

I would suggest you check the warning under mail at the link and fix your domain registration so it is correct. Starting with checking the A records for the domain in DNS.

Then you will be able to use mail as intended and get it delivered. Cut price hosting all to often gives you exactly what you pay for. Web site hosting is not the same as buying a toothbrush. Web hosting is complex and technical. Pay professionals, unless you want to spend a lot of time learning all about how the web works and the checks and balances used to make mail flow and inhibit spam.

In addition to Matt's suggestion: I think your screenshoot shows a logical mistake. Do you have an own mail server in your domain? In your first post you tell us that the mail server is ROHOST, but the screenshoot shows lunacleaning.ro as server.

Maybe the (unused) first smtp server definition works and should be used.

To minimize confusion, you should also change the description from mail address to something meaningful like "OurSMTP"