The Mail Servers Responded: 5.1.1 Unknown Or Illegal Alias

Asked By 780 points N/A Posted on -

My warm greeting to all techyv people , my post here is about my bad experience in sending an email using Thunderbird, an error will occur during sending mail, the error message goes unknown or illegal alias, please check the message recipients and try again.


An error occurred while sending mail. The mail servers responded: 5.1.1 unknown or illegal alias:

Please check the message recipients and try again.




As I read on the error message above, I am wondering why I got an error message regarding the address of the recipient, I am pretty sure its correct, it has been saved on my contacts, and I managed to send some email on past days using same address, that is why I am wondering on what went wrong this time, please share some comments on this issue, thanks.  

Best Answer by techduck_1439
Best Answer
Best Answer
Answered By 0 points N/A #123628

Failed in sending email using Thunderbird could be as follows:

  • Thundebird settings is incorrect – in checking:
  1. Go to Tools menu
  2. Select Account Settings (you can see a list of settings in your account here)
  3. To edit incoming server name, click Server Settings
  4. Click Outgoing Server, for SMTP server to be changed (choose the STMP server you want)
  5. Click Edit

– You can follow above steps if provider does not blocked Port 25.

Let's assume, Port 25 is not blocked , the "Description" will be the name you gave your account. And under the "Security and Authentication" Username and password should be checked. The same username should be used in the "User Name dialog box", as in incoming server (example, your email add, [email protected])

  • Internet Service provider blocked port 25 – Port 25 is a channel used as communication between the server and client. Reason of blocking is to cut down the level of spam that is sent on their network.

To check:

Run Command: START/ Run/ type cmd/ Press ENTER/ then type  this telnet 25 (I assume you are window user). Look for the message. 

If message is connection refused you should use other port for SMTP.

Answered By 0 points N/A #123629

Hello Mary,

That error may be occurring because Thunderbird is not able to parse through the local mailing lists for the email addresses. It appears like Thunderbird in only sending the name of the local mailing lists to the server. And it also appears like the server is not able to recognize the mailing list that is being send and as a result generating the error. The reason to that is because Thunderbird has been set to be local to the machine.

Note that for Thunderbird to work properly it should be able to parse through all sublevel mailing lists for all email addresses before it finally sends to the server.





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