fix SMTP output duplicate headers

[apsega]

Currently, SMTP output will duplicate To:, From:, Subject: headers and due to that services like Gmail are blocking incoming emails. For example:

/kind bug
/area outputs

[Issif]

It's a remainder of the previous version of the lib I used for SMTP. I missed to change that thanks.
The Subject is not there anymore, are you sure it works as expected?

[apsega]

I've just built from the master branch and checked locally, there are still duplicate headers (To: and From:) and email didn't went through. Added additional debug log and it shows (deducted some parts for privacy):

2023/06/22 08:18:37 [DEBUG] : body :
To: <...>@gmail.com
Subject: [Warning] 06:54:15.354304000: Warning Attach/Exec to pod (user=<...>@gmail.com pod=<...>-7fbf9995b-nvcfz ns=namespace action=exec command=/bin/bash)
From: falco@email
From: falco@email
To: <...>@gmail.com
Date: Wed Jun 14 06:54:15 +0000 2023
MIME-version: 1.0
Content-Type: text/plain; charset="UTF-8"

[Issif]

I've just built from the master branch and checked locally, there are still duplicate headers (To: and From:) and email didn't went through. Added additional debug log and it shows (deducted some parts for privacy):

2023/06/22 08:18:37 [DEBUG] : body :
To: <...>@gmail.com
Subject: [Warning] 06:54:15.354304000: Warning Attach/Exec to pod (user=<...>@gmail.com pod=<...>-7fbf9995b-nvcfz ns=namespace action=exec command=/bin/bash)
From: falco@email
From: falco@email
To: <...>@gmail.com
Date: Wed Jun 14 06:54:15 +0000 2023
MIME-version: 1.0
Content-Type: text/plain; charset="UTF-8"

I agree your change, but you also remove the Subject field, are you sure it's OK? Removing to and from, it's good to me.

[apsega]

@Issif you were right. Adding back Subject field.

[poiana]

LGTM label has been added.

Git tree hash: ebe91c33f7b58496572c50327d1b30760a757c81

[poiana]

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