Almost always, one of two things has happened: the email went to their junk folder, or the email address on their record doesn't work. Mucka can tell you which.
Check their record first
Open the customer's record and look at Communications. Every email Mucka has sent them is there, newest first.
- "Delivered" under the email: it reached their email provider. Ask them to look in their junk or spam folder, and to search their inbox for your business name. Emails come from your business name at
hello@mail.mucka.ai, so it's worth telling them to look for that. - "Didn't reach them, check the email address" in red: the address doesn't work. Their email provider told us the mailbox doesn't exist, so the email never arrived and nothing further will until the address is fixed.
You can also just ask Mucka: "Did Sarah Harper get her invoice?"
When Mucka won't send to an address
If an address has already bounced, Mucka won't send to it again. Instead you'll see a message like:
We can't email sarah.harper@example.co.uk. Their email provider says that address doesn't exist, so it wouldn't reach them.
That's on purpose. Once an address has bounced, our email service stops delivering to it, so the email would look sent and never turn up. Refusing it means the invoice doesn't get marked as sent when it wasn't.
To fix it: ring the customer, check their email address (look for typos like .con instead of .com, or a missing letter), then update it on their record with Edit. Send again and it goes to the new address.
If the customer is sure the address is right and it still won't send, raise it with us through Get support in the app and we'll look into it.
You'll be told when it happens
When an email to a customer bounces, Mucka puts an Email not delivered heads-up in your Actions, naming the customer and the address. You only get one per address, however many emails bounce.
This covers emails sent from 24 September 2026 onwards. Older emails in the Communications list just say they were sent, even if they didn't arrive. The check before sending covers older bounces too, though.
What it can't see
- Emails sent from your own connected mailbox (if you've linked Gmail or Outlook in the Inbox). Those go out through your own email account, so a bounce comes back to your own inbox rather than to Mucka.
- Whether they actually read it. "Delivered" means their email provider accepted it. It can still have gone to junk.