Normally Thunderbird will check mail when it first opens, and every 10 minutes thereafter. You can set Thunderbird to only retrieve mail manually in "Account Settings".

You can go into Account Settings using either:

• application menu > Options > Account Settings
• tap the Alt key > Tools menu > Account Settings

Then click into the Server Settings category to deselect automatic checking.

The method mentioned above works for POP accounts, but if the account is IMAP and the IMAP server supports the IDLE command, I think you will receive notifications as soon as they arrive at the server, independent of the settings for checking mail.

http://kb.mozillazine.org/IMAP:_advanced_account_configuration#Features

Thanks! I'm new to Thunderbird.

More good information, and much appreciated!