It seems that it is Microsoft is deliberately causing the 'bouncing' of a percentage of emails if using Microsoft Outlook under SMTP - all other email clients are currently unaffected. It is a security restriction that they have created to get Windows TLS email security pushed to a later version (TLS 1.2). Note that this ONLY affects the use of SMTP button and not the Outlook one.
"Exchange Online will begin rejecting a small (unspecified) percentage of attempts to make SMTP connections using TLS 1.0 or 1.1 and issue this error message -
421 4.7.66 TLS 1.0 and 1.1 are not supported. Please upgrade/update your client to support TLS 1.2. Visit https://aka.ms/smtp_auth_tlsThis is a temporary error and clients can retry the connection. Exchange Online will block only a small percentage of connections it?s likely that the next attempt to connect will succeed. However, that small percentage of declined connections will increase over time to gradually make it more painful for clients using the older protocols to connect to Exchange Online. As Exchange Online declines more connections, clients will experience delays in transmitting email. In some cases, depending on the client?s error handling, email might not get through until someone updates the client to handle frequent retries (or upgrade the client to TLS 1.2."
The source code of the sent emails from Sameday Express indicate that they have ALL been sent under TLS 1.2
Until this annoying situation is resolved we suggest using ?relaying? to get around the issue.
We opened up a Google gmail account then linked our email address to it. We then used the gmail SMTP server settings instead of Outlook and not a single email ?bounced?.
Other relaying systems should work as well, such as smtp2go.
See our PDF on how to use gmail in Downloads on our web site - Outlook_SMTP_TLS_1_2.pdf
Norman
