Thunderbird in Ubuntu sometimes NO email notification

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http://askubuntu.com – Hi Im using Ubuntu 11. I am using thunderbird as my email client. When I have a new email received, ubuntu gives me email notification which appears in my screen. Sometimes also, there is no email notification even though i received an email. This caused me delay in reading fresh emails. Whats the reason for this? Is this a bug in Ubuntu? Whats an appropriate solution? Any help would greatly be appreciated. Thanks (HowTos)