Why bulk e-mails to delegates get blocked or bounce
Why an event organiser's e-mails to large delegate or exhibitor lists bounce or get blocked, what the error codes mean, and how to send them instead.
By Achekam · · 4 min read · For Events businesses in South Africa, Nigeria, Kenya
Three days before the conference, the organiser sends the final joining instructions to every delegate: venue map, parking, registration times. It goes out from the events manager's mailbox, with the whole list pasted into the address line. Within minutes, a stream of "undeliverable" messages arrives. By lunchtime, the events manager cannot send any e-mail at all, including the invoices exhibitors are waiting for.
It is an avoidable problem. Mail systems are built to stop exactly the pattern that a big send from a personal mailbox resembles, and they do so with specific, documented limits. Understanding the error messages tells you what went wrong; a few changes in how you send stop it happening again. This article uses Microsoft's documentation for Exchange Online to explain the main causes.
Too many recipients from one domain in one message
Microsoft says Exchange Online returns error 4.5.3 when a message has more than 200 SMTP envelope recipients from the same domain. The sending server must break the recipients into smaller chunks and resend.
For an organiser, that limit is easy to reach when a large corporate client sends a whole department to an event. Every delegate shares the client's domain, and a single message to all of them crosses the threshold. Splitting the send helps with this specific error, but it is a sign that the message should not be going from a personal mailbox in the first place.
Microsoft's article on e-mail non-delivery reports and SMTP errors in Exchange Online lists each error code, what causes it and how to fix it, and is worth bookmarking for whoever looks after the organiser's mail.
A test domain with a daily cap
Microsoft also says onmicrosoft.com domains, sometimes called MOERA domains, are intended only for testing and are limited to sending to 100 external recipients in a rolling 24-hour period. It advises organisations to buy and migrate to a custom domain for sending to external recipients.
A small events business that set up Microsoft 365 quickly may still be sending from an address ending in onmicrosoft.com. Every delegate, exhibitor and supplier counts towards that cap. Once it is reached, nothing more goes out to anyone outside the business until the 24 hours roll on. The fix is a proper domain of your own, set up before the next event season, not on the morning of a send.
Blocked for spam, often after a compromise
Microsoft says error 5.1.8, "bad outbound sender", means the account was blocked for sending too much spam, typically because it was compromised by phishing or malware. It adds that hitting the 5.1.90 or 5.2.2 sending limits can also indicate a compromised account being used to send spam.
That is the scenario in the opening: the events manager's account stops sending altogether. Two possibilities need checking straight away:
- The large send itself looked like spam to the mail system.
- The account really has been compromised, and someone else has been sending from it.
Treat the second possibility seriously. Change the password, check the account's recent sign-ins and sent items, and look for forwarding rules nobody set up. An events company's mailbox holds supplier bank details and client contacts, which makes it worth protecting.
SPF, DKIM, DMARC and forwarding loops
Some messages are rejected not because of volume but because the receiving system cannot confirm they are genuine. Microsoft lists the causes of DMARC failure as missing or incorrect DMARC or DNS records, including alignment and missing SPF; a missing DKIM record or an unsigned message; and forwarding that breaks SPF or DKIM. It notes that error 5.7.23 means a message was rejected for a Sender Policy Framework violation, and advises checking the domain's SPF record if the Received-SPF header shows anything other than pass.
Microsoft also says that by default Exchange interrupts an e-mail loop after 20 iterations and sends a non-delivery report with error 5.4.6 or 5.4.14, and that such loops usually come from configuration errors such as automatic forwarding rules.
For an organiser, the practical checklist is:
- Ask whoever manages your domain to confirm SPF, DKIM and DMARC records are set up and correct.
- Make sure any service that sends e-mail on your behalf, such as a ticketing platform or a mailing tool, is included in those records.
- Review automatic forwarding rules, especially on shared mailboxes like info@ or bookings@.
Send to lists through a mailing tool, and check what reached customers
The underlying fix for bulk sends is to stop using a personal mailbox for them. In Achekam, a business can keep a mailing list in its CRM, design newsletters and send or schedule campaigns, with an unsubscribe link in every letter. A lead marked "Not interested" is added to the list automatically. Joining instructions, programme updates and exhibitor briefings belong there, sent as a campaign rather than as one message with hundreds of addresses.
Invoices and account e-mails are a separate stream, and the questions there are whether each one went out and whether the customer can pay it. An issued invoice can be downloaded as a PDF on the business's letterhead and e-mailed from the invoice, with or without a pay link, and a failed send is reported as not sent. Credit control is on for new workspaces and once a day e-mails overdue reminders and a monthly statement to customers with a balance; it never charges anyone, and it records whether each e-mail went out. Both depend on the workspace's e-mail settings working, so a blocked or misconfigured sending account shows up there as failures. The meaning of specific bounce codes on invoice e-mails is covered in invoice e-mail bounce codes, and proving an invoice was delivered in evidence that an invoice was delivered.
After the next big send, open the list of invoice e-mails marked not sent, and treat any cluster of failures as a prompt to check the account and domain settings.
General guidance only, not advice on your own circumstances. Tax and payroll rules change: check the current position with the authority or your adviser.
