Mail disappears after it has downloaded in Mozilla Thunderbird

About IMAP IMAP stands for the Internet Message Access Protocol. It enables a mail client (such as Thunderbird) to access messages stored on a mail server. It is more modern and fully featured than POP (Post Office Protocol), which is the other major protocol for accessing mail messages. Most email providers now support IMAP. To see whether your account uses IMAP or POP, check the Server Settings page in the Account Settings... for more info.
https://support.mozilla.org/en-US/kb/imap-synchronization

re-download it from here;

Mozilla Thunderbird - Download

https://mozilla-thunderbird.en.softonic.com/Mozilla Thunderbird, free and safe download. Mozilla Thunderbird latest version: Excellent mail client from the makers of Firefox. Thunderbird is a desktop email ..
Mozilla thunderbird Google Search

1)In Mozilla Thunderbird, from the Tools menu select Account Settings.
2)Select Email account, and then click Next.
3)Enter your name and e-mail address.
4)Select POP or IMAP as the type of incoming server you are using. 5)Your incoming server is pop.example.net for POP, or imap.example.net for IMAP. Click Next.
6)Enter your e-mail address for the Incoming User Name, and Outgoing User Name. Click Next.
7)Enter a name for your email account and click Next.
8)Verify your account information and click Finish.

If you use Outlook, you can try different mail software. Thunderbird from Mozilla (Firefox is browser from them) is good one. When I have some problem with Outlook Express (it is default for windows from Microsoft) I just install Thunderbird, and lot of problems disappear. Microsoft has Outlook (from Office suit) and it can work better then free Outlook Express. But I see some Office with problems and after update its problem is disappear too. I think that Microsoft sell not finished stuff. Maybe because of piracy? Some viruses can fool you too. AVG is good free antivirus...

Step 1. Go to the Gmail web interface and click the Settings link at the top right corner of the page. Look for the "Forwarding and POP/IMAP" tab and click the link.
Step 2. Now you need to enable either POP or IMAP. For most people, POP will work just fine and it's the simpler of the two. Just select the "Enable POP for all mail" option. You'll also notice that you can control what Gmail does when your desktop client connects under the "When messages are accessed with POP" setting. Choose "keep Gmail's copy in the Inbox" since we're just using POP for backup purposes.
Step 3. Now startup Thunderbird, enter your username (the bit before @gmail.com in your e-mail address) and Google password. Thunderbird will begin downloading your e-mail, creating a local backup. There's no need to mess with Thunderbird's settings or since you're just doing this for backup purposes.

are you trying to transfer Windows mail to Thunderbird OR thunderbird to Windows Mail?

• Thunderbird http://kb.mozillazine.org/Importing_and_exporting_your_mail
• Window Mail http://email.about.com/od/mozillathunderbirdtips/qt/et_export_to_oe.htm
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MozBackup is a simple utility for creating backups of Mozilla Firefox, Mozilla Thunderbird, Mozilla Sunbird, Flock, SeaMonkey, Mozilla Suite, Spicebird, Songbird and Netscape profiles.
It allows you to backup and restore bookmarks, mail, contacts, history, extensions, passwords, cache etc. It's an easy way to do Firefox backup, Thunderbird backup ...

Please check the pop setting and port number correctlly then it will work fine.

Firefox without a doubt is the best browser. it's fast, safe and there are plenty of add-ons available for it. you can go here to download --

http://www.mozilla.com/en-US/products/download.html?product=firefox-3.0.5&os=win&lang=en-US

as far as e-mail --
if you want web based e-mail you could just use your internet provider. or go with Hotmail.

if you want a Desktop Aplication - thunderbird is popular and comes from the same people as firefox

Download thunderbird here ---

http://www.mozilla.com/en-US/products/download.html?product=thunderbird-2.0.0.18&os=win&lang=en-US

Hope this helps

Google Gmail Outgoing Mail Server (SMTP): smtp.gmail.com

The Gmail SMTP server requires authentication (use the same settings as for the incoming mail server)

The Google Gmail SMTP Server requires an encrypted connection (SSL) on port 465.

GMail uses POP3 over an SSL connection, so make sure your email client supports encrypted SSL connections.

Google Gmail Incoming Mail Server (POP3) - pop.gmail.com (SSL enabled, port 995)Outgoing Mail Server - use the SMTP mail server address provided by your local ISP or smtp.gmail.com (SSL enabled, port 465)