Many factors can contribute to your email not being delivered. Here is a common list of things that can affect email delivery:
THROTTLING
Some email services can not handle a lot of email sent to them all at once. In order to maintain the stability of their networks, they throttle (slow down) the number of emails they receive from a given domain/IP address. This can sometime cause delays, but as long as the email is not SPAM it will usually get through within a few hours. If your email hasn't bounced and you have not received your email in 4 hrs, let us know so we can find out what's going on.
CONTENT
Not only is it important that the content you are sending to your recipients is relevant, but that it will not be caught in spam. You may run SpamAssassin to help you scan for any possible issues with your email that may cause it to be marked as spam, such as key words and balancing your image-to-text ratio.
MAKE SURE YOU ARE RECOGNIZED AS A LEGITIMATE SENDER
Because spammers often masquerade as legitimate senders (claiming that their emails are coming from a real company) receivers will often look to authentication as a way to see if the sender is really who they say they are. This is especially important when using an email service provider as they may (or may not) be allowed to send on your domain’s behalf.
There are four main protocols in use today (SPF/Sender ID, Domain Keys and DKIM), but because none of these protocols are dominant, any reputable email marketing platform will make all (or as many) available to their clients in an effort to help maximize delivery.