[Mailer] Capitalize sender header for Mailgun #52017
Merged
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This fix resolves the duplicate header issue.
Without a fix, using API & HTTP transport type, emails could leave with double headers, causing problems for some email providers, for example, the problem was reproduced with Gmail:
5.7.1 This message is not RFC 5322 compliant. There are multiple Sender 5.7.1 headers. To reduce the amount of spam sent to Gmail, this message has 5.7.1 been blocked.
Moreover, I found that changes to HTTP transport type were not necessary since this problem could be solved by simply adding a sender, so I reverted everything for HTTP transport type.