If you click into the deferred email list, you may be able to see if it is a particular email provider that is deferring the majority of your email. Different email providers and ISP's have different rules about how they handle bulk messages. 

Reasons for deferring emails include but are not limited to:  

• the receiving server doesn’t recognize your IP, and is afraid you’re sending spam.
• the receiving server doesn’t have any open ports to receive email.
• the recipient’s mailbox is full.
• or some recipients have marked your mail as spam, but not enough for the server to block you. Instead, they will refuse to receive more until they see what the rest of your current recipients do.

Here is a partial list of different providers and some of their policies. (last updated 7/25/12)

Comcast:

Allows 25 simultaneous connections per sending IP address, 100 recipients per message, and they allow a throttle rate based on your sender reputation and authentication.

Comcast Customer Security Assurance should be able to help you: 1-888-565-4329 (M-F 9am-11:30pm ET, S-S 10:30am-6:30pm ET), http://security.comcast.net/get-help/contact-comcast-security.aspx. 

Yahoo!:

Encourages you to try re-sending to deferred emails approximately 4 hours after you encounter this error.

Gmail:

If a mail message has been deferred the sender should try again later.

AOL:

The largest piece of e-mail that an AOL member can accept from or send to is 16 megabytes. This includes the message text, headers and the attachment combined. These sizes can not be changed.

Hotmail:

Hotmail is very strict with spam complaints and they DO NOT bounce emails. They only filter to trash or junk mail. If they get only a few they will filter all email from your account. They will also filter if they detect you are sending to many non-existent accounts, since this is a characteristic of spammers.

Earthlink:

EarthLink blocks IP addresses once they have been associated with spam. EarthLink does not block domains as a part of this process.

• EarthLink has received numerous complaints about the server that handles your email.
• Your IP address has been listed as spam by industry references.