Normally Thunderbird will check mail on a schedule (e.g., every few minutes). You can set Thunderbird to only retrieve mail manually, or you can keep Thunderbird closed until you want it to check mail. To modify the schedule, you can go into Account Settings using either: * application menu > Options > Account Settings * tap the Alt key > Tools menu > Account Settings The click into the Server Settings category to adjust the time interval of deselect automatic checking.