fix: filter Content-Transfer-Encoding from custom headers

[ryanwi]

Problem

The IGNORED_HEADERS list filters MIME headers that the Resend API / Amazon
SES handles automatically (content-type, mime-version, etc.), but
content-transfer-encoding was missing.

The Mail gem includes this as a header field on every message. When
headers_values() reads its unparsed_value, some versions of the Mail gem
return the Ruby constant name (e.g. Mail::Encodings::SevenBit) instead of
the actual encoding string 7bit. This gets passed through to the Resend API
as a custom header and appears verbatim in outgoing emails:

Content-Transfer-Encoding: Mail::Encodings::SevenBit

Email providers like Yahoo flag this malformed header and route messages to spam.

Fix

Add content-transfer-encoding to IGNORED_HEADERS, consistent with
content-type and mime-version which are already filtered.

Test plan

• Added spec verifying content-transfer-encoding is in IGNORED_HEADERS
  and not passed through as a custom header
• All existing specs pass

---

Summary by cubic

Filters Content-Transfer-Encoding from custom headers to prevent malformed values (e.g., Mail::Encodings::SevenBit) from being sent. Aligns with other auto-handled MIME headers and helps avoid spam routing.

• Bug Fixes
  • Added content-transfer-encoding to IGNORED_HEADERS so SES/Resend handles it.
  • Added spec to confirm the header is filtered and not sent as a custom header.

^{Written for commit 217d3a5. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

[drish]

looks good to me, thank you for your contribution @ryanwi