Any mail solution supporting Unicode will let you mail in Bengali. You can of course use Avro to make things easier!
I could not write in Bangla in Yahoo. Probably they don’t support Unicode. I am telling how to write in Bangla in Gmail.
1. Open your Gmail mailbox. Click Settings. Under the option Outgoing message encoding select Use Unicode (UTF-8) encoding for outgoing messages.
2. Now type mails in Bangla. The user you are writing to should be using either Gmail or any other webmail that supports unicode (not Yahoo) or using a email client that supports unicode e.g. Microsoft Outlook.
Needless to say, you can mail in Bangla if you are using Microsoft Outlook or Outlook Express.