If I am upgrading from 11.10 to 12.04 , then will my applications be deleted and also will my already configured mail setting deleted? I have many mails in my Inbox.
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Here is an answer that will help you clear out your ideas about it- http://askubuntu.com/a/138852/46000 – Nirmik Jun 10 '12 at 14:41
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1if any of these answers "answered" your question please accept one of them :) – blade19899 Jul 11 '12 at 15:59
2 Answers
Will my data be erased if I upgrade from Ubuntu 11.10 to 12.04
No:
your mail will not be deleted. At least last time I upgraded, instead of a clean install, all my mail remained(thunderbird).
For your applications when upgrading you have a arrow next to the to be deleted
text. Click it to see which applications will be deleted.

- 26,704
As with all OS upgrades there is an important rule to follow:
Having said that, there is only a small risk that you lose data during an upgrade process. All your private data, e-mails, and application settings will be kept. Only those applications you installed other than from the Ubuntu repositories may have to be reinstalled later, but settings for these applications will be kept too.
This is especially true for applications you installed from a ppa, from a dowloaded .deb package or compiled yourself.