Bump greenmail from 1.6.3 to 1.6.4

Bumps [greenmail](https://github.com/greenmail-mail-test/greenmail) from 1.6.3 to 1.6.4.
- [Release notes](https://github.com/greenmail-mail-test/greenmail/releases)
- [Commits](https://github.com/greenmail-mail-test/greenmail/compare/release-1.6.3...release-1.6.4)

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