In Ometria we use definitions set by ISPs to classify different bounce codes and their meanings.

The logic we use to determine different bounce categories in campaign reporting is as follows:

Soft bounces: 20, 21, 22, 23, 24, 40, 60, 70, 100

Hard bounces: 10, 30, 90

Email bounces: All of the above, plus classes 50, 51, 52, 53, 54

How can delivery rate be 99% if X% of emails are delayed?

Because delayed emails still get delivered, if you look over 24 hours, maybe X% got delayed and re-sent, so delivery can be 99%.

What are delayed emails?

Delays (aka Temporary Failures or 4xx Errors)

When we receive a 4xx rejection from the ISP, we log a "delay" event that includes the timestamp and the attempt count and retry again later.  

If we hit the maximum number of retries (or maximum time period the message is eligible for delivery) and the remote server still has not accepted the message, we will fail the message with a timeout reason – in this case we will log a "bounce" event.  
These types of bounces (those for which the remote server continually rejects the message with a 4xx error) are not usually classified as hard bounces.

Hard Bounces (aka Permanent Failures or 5xx Errors)

When we receive a 5xx response from the ISP, we will log a "bounce" event and determine the bounce classification based on the 5xx response string.  (Bounces classified as "hard" bounces (code 10) are added to the suppression list.)

How long are emails delayed before we do not push them anymore? i.e. people receiving promos after they have expired.

On the first failure the message is sent to the delayed queue and retried 20 minutes later.On the second failure, the retry interval doubles to 40 minutes.On the third failure, the retry interval quadruples to 80 minutes.On the fourth failure, the retry interval multiplies by eight, to become 160 minutes.

Why do 'email hard bounces' and 'email soft bounces' not add up to 'email bounces'?

Because you also have admin bounces and block bounces:
Blocked – These emails were refused because the recipient server determined that the content is spammy, the IP has poor reputation, or a myriad of other filter verdicts. These are rejected at the recipient server level, and never reach the intended recipient.
Admin – These emails were blocked due to a previous unsubscribe, spam/feedback loop complaint or hard bounce.  Admin Failures are a result of our system monitoring and protecting your sending reputation.