I got somewhat further.

When setting

submission_backend_capabilities = DSN 8BITMIME

in 20-submission.conf, DSN works.

But:

The comment says

Part of the SMTP capabilities that the submission service can offer to the

client (as listed in the EHLO reply) depend on those capabilities also

being

provided by the relay server. These capabilities currently are:

- 8BITMIME

- BINARYMIME

- DSN

- VRFY (always returns 252 without support)

By default, the submission service first connects to the relay server to

determine the support for such capabilities before sending the initial

EHLO

reply to the client. If the list of capabilities returned by the relay

server

is somehow unreliable or it is undesirable to start the connection to the

relay server before the first mail transaction is started, the backend

capabilities can be configured explicitly using the

submission_backend_capabilities setting. This is a space-separated list of

SMTP capability names. This setting is only relevant for capabilities that

depend on support from the relay server: including (or omitting)

capabilities

that are not listed above has no effect. When this setting is

explicitly set

to the empty string, none of the capabilities is enabled. To achieve the

default behavior, this setting must be left unconfigured.

However, I do not see dovecot connecting to postfix after connecting to dovecot-login and entering "ehlo ...".

There is no connect to localhost:587 until transmission of the message has started (and too late to issue an ehlo response...)

regards

I got somewhat further.

When setting

submission_backend_capabilities = DSN 8BITMIME in 20-submission.conf, DSN works.

But:

The comment says

Part of the SMTP capabilities that the submission service can offer to

the

client (as listed in the EHLO reply) depend on those capabilities also

being

provided by the relay server. These capabilities currently are:

- 8BITMIME

- BINARYMIME

- DSN

- VRFY (always returns 252 without support)

By default, the submission service first connects to the relay server to

determine the support for such capabilities before sending the initial

EHLO

reply to the client. If the list of capabilities returned by the relay

server

is somehow unreliable or it is undesirable to start the connection to

the

relay server before the first mail transaction is started, the backend

capabilities can be configured explicitly using the

submission_backend_capabilities setting. This is a space-separated list

of

SMTP capability names. This setting is only relevant for capabilities

that

depend on support from the relay server: including (or omitting)

capabilities

that are not listed above has no effect. When this setting is explicitly

set

to the empty string, none of the capabilities is enabled. To achieve the

default behavior, this setting must be left unconfigured.

However, I do not see dovecot connecting to postfix after connecting to dovecot-login and entering "ehlo ...".

There is no connect to localhost:587 until transmission of the message has started (and too late to issue an ehlo response...)

regards