Possible reasons: Check each one.

1. You have reached a maximum quota on server, so until an email is deleted there is no space. Access webmail account and empty the server Spam folder and delete very old emails. Please note: if you have an imap account then it will also remove those emails from the imap folders.

2. Something is causing an interference. This is usually the Anti-Virus scanning opened files. Suggest you make Thunderbird profile folder exempt from scanning.

3. In Firewall, Check Thunderbird is set up as an allowed program.

4. Maybe the account folders are being auto compacted. Suggest you try switching off the auto compact.

• Menu icon > Settings > General
• Scroll to 'Disk Space' section
• uncheck 'compact all folders when it will save over x MB'

Therefore once all is running ok, manually compact the folders on an individual basis.

• Right click on folder and select 'Compact'

The Junk, Drafts and Inbox folders will need more compacting than others because those folders get more emails deleted and moved to other folders.

5. Problem with the index files.

• Menu icon > Help > More Troubleshooting Information
• Under 'Application Basics' - half way - Profile Folders - click on 'Open Folder'

A new window opens showing the contents of your profile name folder.

• Exit Thunderbird now - this is important

If POP account:

• click on 'Mail' folder
• click on 'pop account name' folder

Files that do NOT have any extension are mbox files and they contain emails - do not delete any of those mbox files.

• Delete all the index files - they will have a *.msf extension.

If you have subfolders then you will see folders with a *.sbd extension

• Select each *.sbd folder in turn and delete all the files that have a *.msf extension.

Start Thunderbird. All the index files will get recreated.

Some servers stop access if you have sent more than your limit.

Some servers stop access if you have sent more than your limit.

Actually, this was not the solution in this case; I found a different solution but appreciate the suggestions.

Toad-Hall - While looking through your suggestions, I got a message from my server: "It appears that you might have a device that is checking your email that has the wrong password and keeps trying repeatedly in a short period of time. I did notice that your IP address was ‘banned’ by our system. I’ve released the ban but it will auto-ban again if it isn’t fixed." I discovered that the "Mail" app on one of our computers had a password with too many characters, so it was incorrect but I wasn't able to change it. I deleted the email account from the "Mail" app since we prefer using Thunderbird. So far, so good... Thanks for your responses!