It might be possible that you are sending large number of mails but most of them are falling in junk or spam folder of recipient instead of inbox, probability of opening that mail will be very less.

To prevent this with best exercise,

•            Don’t use false or misleading header information:

 Your “From,” “To,” “Reply-To,” and routing information – including the originating domain name and email address – must be accurate and identify the person or business who initiated the message. 
 

•          Don’t use deceptive subject lines:

 The subject line must accurately reflect the content of the message. 
 

•            Identify the message as an ad:

The law gives you a lot of leeway in how to do this, but you must disclose clearly and conspicuously that your message is an advertisement. 
 

•            Tell recipients where you’re located: 

Your message must include your valid physical postal address. This can be your current street address, a post office box registered with the Postal Service, or a private mailbox you’ve registered with a commercial mail receiving agency established under Postal Service regulations. 
 

•            Tell recipients how to opt out of receiving future email from you: 

Your message must include a clear and conspicuous explanation of how the recipient can opt out of getting email from you in the future. Craft the notice in a way that’s easy for an ordinary person to recognize, read, and understand.
 

•            Honor opt-out requests promptly: 

Any opt-out mechanism you offer must be able to process opt-out requests for at least 30 days after you send your message. You must honor a recipients opt-out request within 10 business days. Once people have told you they don’t want to receive more messages from you, you can’t sell or transfer their email addresses, even in the form of a mailing list.