If the email size is too big then it will be returned.

just because your server will handle eg: 20MB, the receiving server may only handle eg: 12MB.

So did you send a large email?

If the receiving server mail box has reached it's quota and is full then the email will be returned as undelivered. This requires the person who has the account to delete some emails or move them off the server.

If there is a limit on the number of emails you can send in a specified time frame then perhaps your server stopped the send.

You do not say whether the email had any attachments or whether you were sending out one email to several people, in which case how many people are you talking about. Did you just add the email address to a TO field or did you create a mailing list? Can you offer more info?

when you get an error message and need to tell people on a forum about it, it is important to state the whole message word for word as a difference in the wording could indicate a different reason.

"The size of the message you are trying to send exceeds a temporary size limit of the server. The message was not sent; try to reduce the message size or wait some time and try again. The server responded: 4.1.0 <72.168.129.168> Connection MUST authenticate.."

 Is the error message I"m receiving. Have gone into, via the internet, to my ISP and have deleted all messages/trash, and attempted to resent a "test" message; with not text.

Bobrm2: sounds like your smtp settings are not quite rght. Do you use these settings, if no what do you use? make sure you have selected this : Authentication Method: Normal Password

Tools > account Settings > Outgoing Server (SMTP) select the server name.

Outgoing Server name: smtp.windstream.net Username: your full Windstream.net email address port: 25 connection Security: none Authentication Method: Normal Password

Folling your instructions, settings same as ISP's, I did manage to send and receive One test message yesterday.

 Trying this morning all attempts are complete failures; with requests to "enter new password". A password which has not been changed.

 Just now attempted to send a test message three times; receiving the same request for a new password then some how forcing the issue I received the following:

  "An error occurred sending mail: The mail server sent an incorrect greeting:  pacmmta54.windstream.net pacmmta54 4.7.1 - Connection Refused - 72.168.134.151 - Too many connections."

 By the way, I've (using the package manager) have completely un-installed and then then reinstall "Thunderbird".  
 An interesting note; I should have had NO orginal settings or have retained any incoming email messages, correct?, but I did and infact click on the link of your original reply. This message is not being sent from in side Thunderbird, but from Mozilla support web page. Thank you for your assistance.

Bob

Finally; got in touch with "Tier" 2 tech support, my ISP. My computer's address was "Blacklisted". Tech set address from neutral to white listed, then back to neutral, changed the password and everything is OK.

 Someone had hijacked the computer's address and was using it to "Spamm".

Thank yo

Bobrm2: many thanks for the feedback, this may help others. Good to hear all is now ok.