Rediffmail Not Working Well These Days - Mailbox Unavailable

Rediffmail has been acting totally weird for the past couple of weeks.Whatever i do , i just wasn't able to send an email to a person who has a rediffmail id.I get the error saying mailbox unavailable or does not exist.Now im sure the mailbox should exist cause the username exists cause i got an email from that person few days back.And this thing isnt happening with only this person , i wasn't able to send an email to 3 or 4 rediffmail ids.I tried sending from various accounts but i get the same problem.This is not exactly a permanent problem ifeel , cause it works on and off.Like if i wait for a few hours and then try , i probably would be able to get the mail through.But hey this is not how a mail service provider should work right.
At first, i thought this was a temporary issue, maybe some server was down or something.But hey, its been weeks and i get this problem on and off.Its really annoying at times, its getting difficult for me to keep in touch with those people using rediffmail.This is the message( or error message rather) i get when i send an email to a rediffmail account.

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 Requested action not taken: mailbox unavailable (state 14).

God help rediffmail, hope they sort out this problem soon!!