We have verified it is nothing on our sever, as we can login directly on our server webmail service, and on other browser based email logins (gmail). We have verified server settings, port numbers, etc.

I think you have not actually demonstrated anything with your test of web mail. One is a web page, and your issue is with mail.

Personally I think you answered your own question'

It is only happening with one inbox, it's a general inbox that multiple (5-6) users access. What's going on here?

IMAP servers have a limit to the number of concurrent logins that they will support. Generally Thunderbird will use up to 5, apple mail on an iphone will just keep connecting until it is refused. These multiple connections make things happen faster and allow for say an expunge on one folder while message headers are retrieved for another folder and finally a message is displayed because you selected it.

Thunderbird has controls on the number of connections for each account in the advanced button in server settings for the account. But I think your issue is actually more involved. It maybe that the total connections for a single rackspace mail account is 25 (your stated 5 users at Thunderbird's default 5 connections.) This means you will have to reduce the connections per user to stay within the limit, but as you decrease the number of connections things slow down. Then if you get a sixth user then the whole issue come up again, and again for a seventh. I suggest oyu determine exactly what server limit you are working within.

contactus7 said

We have verified it is nothing on our sever, as we can login directly on our server webmail service, and on other browser based email logins (gmail). We have verified server settings, port numbers, etc.

I think you have not actually demonstrated anything with your test of web mail. One is a web page, and your issue is with mail.

Personally I think you answered your own question'

It is only happening with one inbox, it's a general inbox that multiple (5-6) users access. What's going on here?

IMAP servers have a limit to the number of concurrent logins that they will support. Generally Thunderbird will use up to 5, apple mail on an iphone will just keep connecting until it is refused. These multiple connections make things happen faster and allow for say an expunge on one folder while message headers are retrieved for another folder and finally a message is displayed because you selected it.

Thunderbird has controls on the number of connections for each account in the advanced button in server settings for the account. But I think your issue is actually more involved. It maybe that the total connections for a single rackspace mail account is 25 (your stated 5 users at Thunderbird's default 5 connections.) This means you will have to reduce the connections per user to stay within the limit, but as you decrease the number of connections things slow down. Then if you get a sixth user then the whole issue come up again, and again for a seventh. I suggest oyu determine exactly what server limit you are working within.

Thank you for the answer. However, we've been using this mailbox the same way for several years. In fact, the number of users has decreased from 7-8 to 5-6. Not all users are actively using the inbox on Thunderbird, but do have it up in the background. We will try to find out how many total connections a single rackspace mail account can handle...though as stated above, we've had more users in the past.

Also, this morning, one of our users got the prompt first thing in the morning, at which time only two people were actively using Thunderbird, the remaining users didn't even have their computers on yet. Furthermore, one user seems to be getting it more often than others. He is using the most up to date Thunderbird version while others are a version or several versions behind. The user getting the login error has to use a VPN service to alter his IP address and then Thunderbird accepts the login information and he can continue to use it.