Based upon Microsoft Articles published 13.12.2022
We are asked many questions about sending emails via O365 online Exchange.
The first thing to remember is that you and your colleagues sending emails from Outlook, or via an attached web client is very different to a back office system such as Formate eVo sending emails. The second is that strictly speaking online Exchange or O365 is not a 100% like for like replacement of your old on premise Exchange server. Due to the nature of the beast there are usage rules enforced by Microsoft.
The key Microsoft thought is
“Exchange Online isn’t suited to accommodate bulk-mailing scenarios” .
The webpage referenced below has excellent additional information, and some possible alternatives if needed.
Looking at some of the rules;
30 emails in a minute, or 10000 total recipients a day, can all come into play if you are sending several thousands Statements at the end of the month. Even if only sending 60-70 invoices, the 30 in a minute could be a factor.
Formate has minimum send intervals etc which can be used to throttle sending, or multiple connections could be used. But it is good to be aware of Microsoft’s limits and views on the subject.
Microsoft impose the following limits as standard (although we find that application is more variable than defined here). Some limits are hard, and some more flexible.
With thanks to Microsoft; (Article published 13/12/2022)
1 After the recipient rate limit is reached, messages can’t be sent from the mailbox until the number of recipients that were sent messages in the past 24 hours drops below the limit. For example, a user sends an email message to 5000 recipients at 09:00 AM, then sends another message to 2500 recipients at 10:00 AM, and then sends another message to 2500 recipients at 11:00 AM, hitting the limit of 10,000 messages. The user won’t be able to send messages again until 09:00 AM the next day.
2 You can customize recipient limits between 1 and 1000 for existing mailboxes and for new mailboxes that will be created in the future. Edit the recipient limit on existing mailboxes individually or in bulk using the Exchange admin center and customize the default setting for new mailboxes via Remote PowerShell. For more information, see Customizable recipient limits in Office 365.
3 These numbers can vary slightly since the limit is based on number of characters so the number of email addresses will vary depending on how long they are. As a rule of thumb, it is safer to assume that the limit of smtp addresses in ProxyAddresses is approximately 300 addresses to leave room for future growth of the object and its populated attributes.
4 When outbound message volumes surpass the message rate limit, any excess in message submission will be throttled and successively carried over to the following minutes. This will typically not block the sender’s account, but Exchange Online isn’t suited to accommodate bulk-mailing scenarios. For this use case, options 2 and 3 here are recommended instead.
please see for full details and some options
Exchange Online limits – Service Descriptions | Microsoft Learn
or