WithBoundary/SetBoundary messes up multipart-rendering

[wneessen]

Description

While working on the fix for #412, I noticed a false behaviour when working with WithBoundary/SetBoundary. When a fixed boundary is set using one of those options, the boundary for each multipart will be the same. This messes up the mail rendering for multipart mails (i. e. body, alternative part and embed).

While this is not related to the v0.6.0 release (this has been an issue since the boundary-options have been introduced) it gets more of an issue with S/MIME signing, since that code makes use of fixed boundaries.

To Reproduce

Create a multi-prt message with fixed boundary.

Expected behaviour

Each multi-part needs a non-unique boundary.

Screenshots

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Attempted Fixes

No response

Additional context

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