@sap-cloud-sdk/mail-client: Allow OnPremise MAIL destinations without basic authentication so that no user and password is required

[gregorwolf]

Is your feature request related to a problem? Please describe.

OnPremise mail servers might not need any authentication. But with the check in mail-client/src/mail-client.ts#L82 it is currently mandatory to provice a user and password by setting the MAIL destination to the Authentication type BasicAuthentication and maintaining User and Password.

Describe the solution you'd like

At least for destination of Proxy Type OnPremise the @sap-cloud-sdk/mail-client should allow destinations where Authentication is set to NoAuthentication.

Describe alternatives you've considered

The configuration of the internal mailserver can't be changed quickly.

Impact / Priority

Affected development phase: Production

Impact: Impaired

Additional context

Same customer as in #5015 is impacted.

[gregorwolf]

Hi @marikaner / @ZhongpinWang,

can one of you please review?

[ZhongpinWang]

Hi @marikaner / @ZhongpinWang,

can one of you please review?

Hi @gregorwolf , sorry for the delay, and thanks a lot for your contribution. I left some comments in your PR.