Delivery is like going to the postal office and having them accept your payment and 100 letters from you to send to your family and friends. They take the letters and send them to have them processed somewhere else. It isn't at this stage that you know that people will actually receive them. Deliverability is knowing that people received the letter in their mailbox and can engage with it after they are processed.
Your letters (or emails) may end up in the right place, or they may not. There are many logical reasons they may not, such as:
People changed address
The person at the counter doesn't like you and processes them all late
There is a mistake in the address
It got lost when processing
Their mailbox is full, and it ended up flying away or wet from the rain
In the world of email, those are the issues that can be dealt with easily enough (Will share more in order to tackle all those issues in the following posts).
Ensure that you authenticate your domain name with all tools that are sending email with your company's email addresses (All of them!). Ensure you are following all best practices, and don't rely on only the deliverability of the email system you are using. What we should all care more about is deliverability and not delivery. Deliverability takes into account your domain's reputation and the engagement of your subscribers. Just like a credit score takes more than your income into account. Here is a list of a few things taken into account when an inbox provider has to decide what to do with your email:
The number of subscribers you have & and how quickly your volume accrued
Are you authenticated?
How many spam traps have you hit lately?
How engaged are your subscribers?
Are you emailing subscribers who have not engaged even once with you in the last months?
When subscribers add your sender email in their contact list (Safe sender button)
Have subscribers saved your email (starred)?
How "heavy" is your email?
Does the content and writing style resonate with your business and customers?
Are you sending from a email@example.com?
Delivery: You sent your email to your subscribers, and the inbox of your recipients lets you know they received it.
Deliverability: How many emails, from the ones you sent, were accepted by your subscriber's inbox AND landed in the inbox.
So stop wondering why your customers don't engage with your brand if you have worked hard on your strategy and follow all best practices. Maybe, most of them haven't even received the email, to begin with.
Note: Keep in mind that when your favourite email service provider is talking about their delivery rate (usually in the high 90s), they are talking about their own reputation. This doesn't mean that your emails will inbox, just like the bank won't give you a loan if your mother has excellent credit, and you don't.