What is the difference between single opt-in & double-opt-in?

Any Video tutorial course available for learning email marketing?

Does conversion rate alters by "opt-in email address"?

Any tool that handles one time & recurring payments?

Is it worth to buy an email list? How can I do that?

Is “unsubscribe link” very important term in email marketing?

Is there any tool where I can create and send beautiful emails with $0 cost.I want to send approx 200000 emails every month.

Can you name the email marketing companies that run from India and are very successful ?

I don’t want to manage revenue analytics manually. Can any software automatically manage the same?

I want to increase my domain reputation so that whenever I send any emails they go inside the email inbox.I have few questions regarding my domain reputation.How do I track my domain reputation?How do I improve it? 

I want to know the list management factors, can you just explain it in an easy way.

Is there any significant day or time for sending marketing emails?

How can I create signup form?

How can I resend the non-open emails automatically? Any ESPs that provide email campaign tracking?

I'm getting a very less open rate. What practices should I follow in order to increase it?