Mail Merge - some messages does not reach their destination, no error messages

Some messages sent via Word Mail Merge fail to reach the recipient, even though everything indicates that they have been sent. The messages appear in the sender's "Sent Items" folder, and no error messages are generated. Personal information from an Excel spreadsheet is merged into each message. I do not know exactly who or how many of the approximately 250 recipients are not receiving their messages, but I have received a few reports from people stating that they did not receive a message they were waiting for. Furthermore, the messages do not end up in the recipients' junk/spam folders. There does not appear to be any pattern regarding the email address domains. I have also tried sending the message to each person on the recipient list individually—still using Mail Merge, and sending them as quickly as possible, one after another—yet one recipient is complaining that they did not receive the expected message. And if that’s the case, there could easily be others. As long as it behaves this way, the feature is completely useless. The least one expects is an error message. Is this a known issue, and is there a solution for it? I am using the latest version of MS 365 and classic Outlook.