I aware about currently that outgoing mail via any mail client i.e. outlook express, microsoft outlook, mozilla thunderbird is blocked by Airtel Broadband DSL connection in Delhi (India).

I have already complaint to Airtel Customer Care, they said that we are looking this issue. But this problem is not resolved at all.

I appeal to users of Airtel Broadband DSL in Delhi to contact Airtel Customer Care and file thier complaint because I think, they are not taking this issue seriously.

Thanks.
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Is it possible that your MTU is set incorrectly?

My Verizon DSL account, at first, could not send any outgoing email using my Outlook. At first I thought it was SMTP server gone down. However, after careful search I found that if MTU is set at 1500. DSL PPPoE cannot send outgoing mail using that high value.

This MTU is not set by your router or any hardware, it resides in your Windows registry. So I searched for my TCP/IP MTU value and set to 1492 (8 bit lower than the maximum allowed for my PPPoE) and it has been working correctly.

In United States this issue is common if your system was upgraded from Windows 2000 or Windows XP Professional before any SP1 or SP2 installation. Verizon was never aware of this situation since it is individual's user specification. Verizon, like most big companies, assume that your computer is factory set-default. The software provided by Verizon was never used since I manually configured my DSL connection--by entering username and password directly to my DSL modem.

In addition, several computers that I have worked on were missing this MTU values from registries. In this case you must enter the MTU value manually by adding the string.

Article on how to modify MTU value in registry: http://www.winguides.com/registry/display.php/280/