On Wed, Apr 08, 2020 at 02:46:00PM +0200, Erwan David wrote:

> I got this problem sometime ago. It was due to a difference in the
> NTLM used by postfix (I think it comes from dovecot) and the one used
> by MS.

For the record, the change is in SASL, and Postfix uses Dovecot SASL
only when receiving mail.  When sending, Postfis uses Cyrus SASL,
which perhaps the OP upgraded to include XOAUTH2 support.

The SASL mechanisms supported by [smtp.office365.com]:587 are:

    posttls-finger: < 250-BL0PR02CA0022.outlook.office365.com Hello [...]
    posttls-finger: < 250-SIZE 157286400
    posttls-finger: < 250-PIPELINING
    posttls-finger: < 250-DSN
    posttls-finger: < 250-ENHANCEDSTATUSCODES
    posttls-finger: < 250-AUTH LOGIN XOAUTH2
    posttls-finger: < 250-8BITMIME
    posttls-finger: < 250-BINARYMIME
    posttls-finger: < 250-CHUNKING
    posttls-finger: < 250 SMTPUTF8
    posttls-finger: > QUIT

"LOGIN" and "XOAUTH2".  Terminology corrections aide, disabling the
latter via:

> Adding
> smtp_sasl_mechanism_filter = plain, login
> made things work again

is most likely the correct solution, with "login" the one that'll be
used in the case of "[smtp.outlook.com]:587".

-- 
    Viktor.

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