Bugfix/issue 667 mailcore lib

[sosnovsky]

This PR integrates pre-built Mailcore framework as SPM dependency.

close #667

---

Tests (delete all except exactly one):

• Does not need tests (refactor only, docs or internal changes)

---

To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation