One of the most common issues small businesses face is an invoice not being paid on time. But often, the reason is surprisingly simple: the invoice never reached the client in the first place.

This usually comes down to how email works behind the scenes, using something called SMTP — Simple Mail Transfer Protocol. In plain English, SMTP is the system that lets one email server talk to another.

Where Things Go Wrong

• My accounting software, like QuickBooks, uses its own email system to send invoices.
• That system has to “speak” to the incoming mail server belonging to my client.
• If the two systems can’t communicate properly — even due to a small technical error — the email may never arrive.
• I don’t always get a notification that something failed, so I assume the invoice went through.
• The client never sees it… and naturally doesn’t pay it.
• This even happens with my clients who always pay quickly. When a payment doesn’t arrive, it’s often the first sign that the email simply didn’t make it into their inbox.

Email leaves one system and may not reach another system.

A Simple Way to Understand It

If john@domain.com has a mailbox on a server, that server is supposed to put the email into John’s inbox. But if there’s a mismatch, a block, or a technical fault, the email might never get there — and neither of us would know.
The Bottom Line

Before assuming a client is ignoring an invoice, it’s worth remembering: they may not be avoiding payment… they may genuinely never have seen it.
If you run a business, it’s always okay to follow up politely — often you’re just helping the client discover a delivery issue they weren’t aware of.