that part of the message is weird. That looks? like an IPV6 internet address which is "unknown" in DNS in so much as it has no name "unknown". But it is the port that is truly weird. 51018

The IP address appears to resolve to a city of Ville in France. But I have no idea what could be using Port 51018 as it is not assigned to any "normal" protocol. It is a dynamic port that I would guess could be used for a VPN internally, or a proxy in a public wifi situation.

My guess is you are trying to send mail from an IP address your service provider does not allow for that purpose.

Hi Matt and thanks for replying. Yes, I am in France. The strange thing is that I have made no change at all to Thunderbird settings and the problem suddenly arose. Settings are: POP Mail server - mail.domainname.com Port 995 Connection security SSL/TLS Outgoing server - smtp.free.fr (Free is the internet provider) Port 465

Hello there

https://support.mozilla.org/en-US/kb/cannot-send-messages

Please read the list sent with your message back ,anything outside this list of solutions may occur less. And even rare in this case.

Greetings Firefox volunteer

Hello there

I would check your stuff carefully for an attempt at a hackers cyber hack on your machine pc.maybe you will find something even stranger than your previous report to us on or from your pc? Free is the Internet , yes that might be a clue to it? Using a hotspot is also using a good antivirus.Traffic on the hotspots WiFi might pass little issues .

You're trying to send from a non-free.fr account through the free.fr smtp server, which might not be allowed unless you add the non-free.fr account as an allowed sender in the free.fr webmail settings. Most services don't allow the sending account to send through an smtp with a different User Name (email address), even if they're on the same domain.

Hello there

The most unexplained situation s can arise and having strange weird port numbers .

Thanks for all your replies and help. My ISP is a company called Free. This is what its website says (translated from French)

To add your free.fr email address to your messaging service (application or software: Outlook, Gmail, Apple Mail, Thunderbird, etc.), you will need the following information:

Outgoing server

The SMTP served used to send your emails from your messaging service is smtp.free.fr on secure ports STARTTLS 587 or 465 or else 25.

Incoming server

The IMAP server used to consult emails hosted on the free.fr server is imap.free.fr on secure port SSL 993 or else 143. The POP server used to transfer your emails to your messaging service is pop.free.fr on port 995 or 110.

Please note I have a domain name for my emails. Have tried ports 587, 465 and 25 for outgoing mail, unsuccessfully. I can receive mail but not send.

This problem occurred around two weeks ago although I have made no changes to settings. Is the above information useful in helping me fix the problem? Thank you for your time.

It is also worth noting that 115.3.1 is exceedingly old, at 1.5 years.

Thanks. I updated to the latest version of Thunderbird but still cannot send email. I should probably find an alternative to Thunderbird as suddenly it isn't working for me. Shame.

Thanks. I updated to the latest version of Thunderbird but still cannot send email. I should probably find an alternative to Thunderbird as suddenly it isn't working for me. Shame.

Let me walk you back to your previous comment which answers your inability to send email.

Please note I have a domain name for my emails. Have tried ports 587, 465 and 25 for outgoing mail, unsuccessfully. I can receive mail but not send. This problem occurred around two weeks ago although I have made no changes to settings.

1. You have a domain name for your emails;
2. Where is the domain hosted?
3. What SMTP server comes with the hosting package?
4. Why are you not using it to send mail for your domain?

You should be using the SMTP server provided by your web hosting provider, not SMTP.free.fr

Odd as it may seem, you will be getting tangled in anti spam tools that prohibit the use of relays to send mail. Such mail routinely fails DMARC, SPF and other anti spam policy frameworks

You are relaying mail for your domain via the Free.fr servers. Free.fr has probably found themselves blacklisted with other major providers for allowing that and have changed policies.

5. Have you configured free.fr as a permitted sender in the MX record in DNS for your domain?

I really can not comment with details from free.fr however because their website offers one of the worst support experiences I have ever been involved with. Information is dumbed down as to be useless, is mostly incomplete and navigating in a language other than french is not for the feint-hearted.

Matt, thank you for your time and trouble. The domain name is hosted by a company called French Connexion at domaine.fr

I changed the primary DNS to free.fr but this hasn't solved my problem. I have switched it back to dns2.french-connexion.com

There is a secondary DNS that is dns40.domaine.fr

I contacted the domain name host for support. Their reply: Serveur de messagerie entrant : dns40.domaine.fr Serveur mail sortant : dns40.domaine.fr Port de messagerie sortant: 25 et / ou 587

I tried changed the outgoing mail server from smtp.free.fr to dns40.domaine.fr port 25 then port 587. It didn't work.

I'm going park this until the weekend. For the moment it makes no sense to me. Thanks again.