Email sent from btinternet to lineone.net get bounced from outlook.com

[gmcalister]

Emails sent from a btinternet address to a lineone.net address get bounced from outlook.com. Specifically, an email sent from <me>@btinternet.com to <you>@lineone.net gets delivered but also gets a "Undelivered Mail Returned to Sender" from "the mail system at host mx-out.tt.xion.oxcs.net" with the message:

<you@outlook.com>: host outlook-com.olc.protection.outlook.com[52.101.73.5]
    said: 550 5.7.515 Access denied, sending domain BTINTERNET.COM doesn't meet
    the required authentication level. The sender's domain in the 5322.From
    address doesn't meet the authentication requirements defined for the
    sender. To learn how to fix this see:
    https://go.microsoft.com/fwlink/p/?linkid=2319303 Spf= Fail , Dkim= Pass ,
    DMARC= Pass [LV8P222MB1062.NAMP222.PROD.OUTLOOK.COM
    2025-06-26T11:20:05.379Z 08DDB1F8330F67F5]
    [PR3P191CA0002.EURP191.PROD.OUTLOOK.COM 2025-06-26T11:20:05.397Z
    08DDB2D0C96BFF30] [AM4PEPF00025F97.EURPRD83.prod.outlook.com
    2025-06-26T11:20:05.424Z 08DDB090B1290755] (in reply to end of DATA
    command)

How did the outlook address get involved? It was not part of any address sent from the email client (Thunderbird).

Thanks for any help.

[licquorice · Accepted solution]

I suspect you have Outlook accounts on Thunderbird which seems to work differently to other clients in as much as it uses a default SMTP server to send rather than associating email addresses with the relevant SMTP server.

Make sure you are using mail.btinternet.com as your SMTP server to send BTinternet mail.