When you send an email to your subscribers via SendGrid (Certain's 3rd Party Email Provider), only a few things can happen to that message between SendGrid and the email recipient's server. Those events are:

• Delivered – the recipient's server accepted the message
• Bounce – the recipient's server denied the message, and the address the message is sent to is suppressed by SendGrid moving forward.
• Blocked – the recipient's server denied the message, and the address is not suppressed by SendGrid moving forward.
• Deferred – the recipient's server delayed acceptance of the message

Let’s take a quick look at what each of those events means so you can glean a better picture of your email delivery performance.

Delivered

A common misconception that I often run into is that a delivered message is a message that made it to the inbox. When a message is registered as delivered, the receiving server has accepted the message from SendGrid. At that point, the receiving server still has to decide what to do with that message.

Most often, the two options facing that server are to deliver the message to the intended inbox or to send it to the spam or junk folder. Less frequently, a receiving server might choose to simply drop that message, akin to the receiving server deleting it. In those cases, the message will not be findable to the recipient, even if they look for it in their spam folder. Dropping a message is rare, though it can happen.

Almost always, the decision about where to put the message comes down to the sender's sending reputation and the content being sent. Senders who generate excessive spam complaints from recipients, or senders sending content known by inbox providers to generate high rates of spam complaints, are much more likely to see their emails not make the inbox. Having a high percentage of delivered emails is great. However, simply getting emails delivered is only part of email success.

Bounce

While some in the industry use terms like “hard bounce” and “soft bounce” to describe messages that were refused by the receiving server, SendGrid’s terminology separates returned messages into bounces and blocks.

Bounces occur when the receiving server returns a code indicating that the reason for the refusal is a permanent issue with that server or recipient address. The most common reason an address will bounce is that the address in question is not valid.

Blocked

Similar to what you may have heard referred to as a “soft bounce,” blocked emails are ones where the receiving server returned to us a reason for refusal that indicates a non-permanent rejection for that address. In short, it is a rejection of that message and not an indication of the quality of the address being sent to.

Common reasons a message might be blocked are if your sending IP or domain has been list denied, the content of your campaign contains elements the inbox provider deems “spammy,” or there was a technical issue that occurred between the two servers at the time you attempted to send the campaign.

Deferrals

A deferred event, or deferral, is simply an event SendGrid has received back from the receiving server that tells us that the receiving server has temporarily limited access to their system. If you are like me and old enough to remember life before call waiting and cell phones, you can think of this as a busy signal. 

It does not mean that your message will not be delivered. Rather, it signals that your message will not be delivered immediately. This can happen for a variety of reasons. Some of the most common reasons a message might be deferred at an inbox provider are that the inbox provider is seeing too many spam complaints for your email that has already been delivered or that the receiving server is having technical issues at that time.

When SendGrid sees your mail being deferred at a particular inbox provider, they will continue to try to deliver that message for you for up to 72 hours. If a message to a recipient is deferred for more than 72 hours, that deferral will turn into a block. Otherwise, it will result in a delivery event once the receiving server has accepted the message.

Keeping an eye on these events can give you even more insight into what is happening with your emails as they journey from the Certain Platform to SendGrid to the recipient’s inbox. If you’d like to dive deeper into your email delivery health, check out SendGrid's  Email Deliverability Guide.

Source: https://sendgrid.com/blog/delivered-bounced-blocked-and-deferred-emails-what-does-it-all-mean/