Sounds like you are using a Mailing List.

In the 'Address Book', select the 'Mailing List' name to see list of Contacts. Please check that all of the contacts in the list have a valid email address. If one or more contact has no valid email addess then the Mailing List will fail. Suggest you add yourself to the Mailing List, then if you receive an email, you know it did send correctly.

How many contacts are in the 'Mailing List' group ?

re: The email appears in my Sent box Is this a POP or IMAP mail account?

If the message appears in your Sent folder then Thunderbird has handed off message to the SMTP server and Thunderbird is no longer in control of the email. It is possible that a spam filter on the servers between you and that recipients is identifying the messages as possible spam and blocking their delivery.

Are you sending in both Plain Text and HTML mode ?

• Menu app icon > Preferences
• select 'Composition'
• click on 'Send 'Options' button
• select 'Send the message in both plain text and HTML'
• click on 'OK'

If sending the 'Mailing List' group in a Bcc field then add your own email address in the TO field, as some servers may assume if only Bcc is used it might be spam.

What is in the content ? Lots of links ?

What server are you using to send email? gmail? comcast? etc

I selected each recipient separately, rather than a mailing list. And I did send it to myself. 48 recipients. I’ll have to check about POP or IMAP, and I tried Bcc and TO for sending. Probably plain text, no links. Suddenlink server.

I'm also wondering whether you are close to your server limits or server quota.

For example: If server quota is near full, it can only receive or send if there is space on server. If you have a POP account, it is possible you chose to 'keep messages on server' and maybe 'until I delete them' and now the server is reaching quota limits. Every time you delete a message, it creates a bit of space, so allowing a few emails in or out, but it does not have the space to allow sending or receiving of large amounts.

You can also have the same problem with an imap account, but imap accounts do offer quota information - right click on folder in Folder Pane and select 'Properties' - it will offer a 'quota' tab. So you are more likely to know about it.

Logon to the webmail account using a browser and see what is stored on server. If you use a POP account them accessing server and clearing out old stuff would be a good idea. Please note: if you use a POP account in Thunderbird then you may need to delete a load of old emails off server and make sure the server spam and trash folders gets emptied. if you use IMAP in Thunderbird and you delete emails off server then you will lose them in Thunderbird as well, because imap can only show what is on server.

If you were sending too many emails or too much in size to effect bandwidth then you think you would get a bounced back email from suddenlink server saying something like Connection limit reached or similar. Have you tried sending a smaller batch eg: 6 recipients in one email?

re : Probably plain text If you created the email in plain text, you would not have any formatting options in the 'Write' compose window and also you would have switched off the 'Compose messages in HTML format' in the Account Settings.

But, if email was created using HTML - you can see a formatting bar available and the send options do not allow it to be sent as Plain Text and HTML you might have some issues if some people only want HTML or only want plain text. Hence why I advised on what settings to choose.

It would be worth checking to see if your IP address is getting flagged as spam by the Suddenlink Server. It might explain issue if server is flagging as spam and simply not offering delivery. Try these links: https://www.spamhaus.org/lookup/

https://whatismyipaddress.com/blacklist-removal

https://mxtoolbox.com/SuperTool.aspx?