You are probably using accented characters in the email address. The server you are trying to send through cannot handle non-ASCII characters.

From the Wikipedia article - "ASCII":
The characters encoded are numbers 0 to 9, lowercase letters a to z, uppercase letters A to Z, basic punctuation symbols, control codes that originated with Teletype machines, and a space.

This is a common problem with some older servers. Check the email address. You might be able to send it if you just change the accented characters to unaccented ones. No guarantee. If the accents are necessary, you will have to find a different server to use. (gmail?)

TB-38.2 Win10-PC

I'll look at it and see what is a problem. The thing is I have sent to all these people before with no problem. Is Mozilla an old server? I thought it was recent. MM

Mozilla Thunderbird is not a server.

This problem does not lie with Thunderbird. Somewhere along the path that your email takes, there is/was an old server in use. Almost all public servers are capable of handling far more extensive character encoding than ASCII alone. Maybe someone had an issue with a server and put an old one in service for a short time, until repairs could be made.

The message that you received about the non-ASCII characters should give some insight to the server that refused the mail.

TB-38.2 Win10-PC

I have a similar problem, when TBird upgraded to 38.1, I have been getting the error below:

There are non-ASCII characters in the local part of the recipient address

I use the program to send out a weekly newsletter to my customers, and now I am getting that error when trying to send a mass list.

Prior to TBird v38.0, I could send all of the addresses in one e-mail, but since the 38.0 update, I have had to break them up into and send 4 separate e-mails, now since 38.1, I got the non-ASCII error. No one seems to know, or care to fix it.

Worked fine before the updates, now not so much.

Have you figured out how to get the block removed? MM

Not me....still doesn't work.

With reference to the first post in this string, I have the same problem. My address list which has worked perfectly swell in Outlook Express for many years, does not work when imported into THUNDERBIRD because of the "non-asciii character" problem. The presentation of the addresses look like perfectly ordinary alpha or number characters to me and I can't see anything that looks even slightly "non-ascii", but they still will not send. Any suggestions.

I am using TB 38.5.1 with Ubuntu 14.04.3 and have this problem.. I have no non-ascii characters in my local email addresses. I have checked the mailnews.force_charset_override setting in Config Editor, and it is set to false. The checkbox “Apply encoding to all messages in the folder..." is NOT checked in Properties. So how do I send an email to multiple recipients?

I found the issue for my situation. I keep people in my address books who do not have email addresses. Some of these address books are quite large since I manage communications with my graduating classes and military units. When I need to send an email to everyone, I just do BCC's to everyone. In the past, the fact that people without email addresses were included caused no problem. Now they do. I must delete each of these from the send to list or select each individual. This is a terrible regression in functionality. It seems a simple thing to simply ignore those who do not have email addresses rather than to try to send an email to no email address. It worked that way in the past. Why not now?

I have the same problem with Thunderbird 38.6.0 when trying to send to a group set up in Mac OS X (El Capitan) Address Book. None of the addresses, nor the mail group name, contain non-ASCII characters.

This issue NEEDS TO BE FIXED; however, I found a workaround that might be useful to others with the same problem.

Recreate the mail group in Thunderbird's address book (give it a different name to avoid confusion with your OS X Address Book group name). It is very easy to add addresses as a choices list appears using type-ahead as you start entering any part of a recipient's name or e-mail address. Sending to the T-bird address book group (containing exactly the same e-mail addresses as the one in OS X Mail's Address Book) appears to work fine.