If you email them very often, they will put your email on filter.Dont know. They say they take around 28 days to respond to emails but people are getting responses very quickly these days. I always emailed them.
If it's a really urgent, you can right urgent in the subject line.however,it is on their discretion to deem your email " urgent". And reply accordingly!
Look what VO says,
IMPORTANT NOTICE: The subject line of this email and the initials included in the subject line are essential to avoid delays in processing requests and responses. Please include the initials written in the subject line of this email, in all future correspondence with us. For email communication, please use the “Reply to Sender” option of your email software which will keep the subject line intact in your response. For postal communication, please include these initials on the envelope, and highlight prominently on the cover letter.
Now it's clear, you follow the same email for further correspondence. Always use reply option rather than compose a new one.
I hope this will help.