Error sending smtp emails - server error after RCPT TO

[#1]

For the last few days I have been having problems sending emails from my Thunderbird email client.
( I'm an old force9 customer )
It has been working for many many years with no real issues, but now it gives me the message:-

Sending of the message failed.
An error occurred while sending mail. The mail server responded:  
Internal server error.
Please check the message recipient "<cut>" and try again.

I've cut the actual email address I'm sending to for privacy:)

But it does not matter who I try and send an email to I get the same error

I've checked Thunderbirds error log and I can see that it fails after sending the RCPT TO: command.

Any suggestion ?

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Messages from the Thunderbird log:-

mailnews.smtp: Connecting to smtp://relay.force9.net:25 SmtpClient.sys.mjs:132:19
mailnews.smtp: Connected SmtpClient.sys.mjs:429:17
mailnews.smtp: S: 220 mail.enmail.co ESMTP ready

SmtpClient.sys.mjs:452:17
mailnews.smtp: C: EHLO [192.168.1.207] SmtpClient.sys.mjs:677:19
mailnews.smtp: S: 250-mail.enmail.co

250-AUTH PLAIN LOGIN

250 STARTTLS

SmtpClient.sys.mjs:452:17
mailnews.smtp: Possible auth methods: SmtpClient.sys.mjs:959:17
mailnews.smtp: C: MAIL FROM:<<cut>> SmtpClient.sys.mjs:677:19
mailnews.smtp: S: 250 2.0.0 OK

SmtpClient.sys.mjs:452:17
mailnews.smtp: MAIL FROM successful, proceeding with 2 recipients SmtpClient.sys.mjs:1224:17
mailnews.smtp: Adding recipient... SmtpClient.sys.mjs:1229:17
mailnews.smtp: C: RCPT TO:<<cut>> SmtpClient.sys.mjs:677:19
mailnews.smtp: S: 451 4.3.2 Internal server error

[#2 · Accepted solution]

@dallday 

Are you on or off the plusnet network?

I would suggest trying port 587 if you are off network

[#3]

Put authentication on the SMTP server.

[#4]

I'm connected via my router on the plusnet network.

The connection had worked for ages but I understand stuff is changing with the Plus.net emails so I'll try changing to use the 587 port 

[#5]

Decided that It would be a good idea to use authentication even though I'm on their network.

Once I got the correct relay server it now works 

Description:     use login
Server Name:     relay.force9.net
Port:     587
User Name:     username
Authentication method:     Normal password
Connection Security:     STARTTLS

where username is is my force9 username not an emails address 

(sometimes I get confused as to when to use just the name and when to use the full email address)

anyway it seems to be working now 

Seems port 25 does have a problem but 587 works fine

Thanks for the nudge to check ports etc., 

David

[#6]

Sorry I thought that switching to port 587 and using authentication was the fix as my email client is working now 

BUT 

The wifes email client is still failing when I use the same settings 😰

I need to check further and report back

David

[#7]

Okay an update and reboot of the linux box and it now seems to be working

Thanks 

David