Update integration test environment to use proper mail server to test 100% mail

[mxsasha]

Currently we use mailhog, which does not support TLS at all. Migrating to a proper mail server would allow us to test this better in the integration tests.

Recently, I wrote a postfix configuration that meets 100% TLS, even in the new sslyze branch, so documenting it here for when we get to this issue.

compatibility_level = 3.8

tls_high_cipherlist = TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-ECDSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA
tls_preempt_cipherlist = yes

smtpd_tls_cert_file=fullchain.pem
smtpd_tls_key_file=privkey.pem
smtpd_use_tls=yes
smtpd_tls_security_level=may
smtpd_tls_auth_only=yes
smtpd_tls_protocols = >=TLSv1.2
smtpd_tls_mandatory_protocols = >=TLSv1.2
smtpd_tls_ciphers = high
smtpd_tls_mandatory_ciphers = high
smtpd_tls_eecdh_grade = ultra
smtpd_tls_loglevel = 1
smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_tls_session_cache
smtpd_tls_received_header = yes

smtp_tls_note_starttls_offer = yes
smtp_tls_protocols = >=TLSv1.2
smtp_tls_mandatory_protocols = >=TLSv1.2
smtp_tls_ciphers = high
smtp_tls_mandatory_ciphers = high
smtp_tls_exclude_ciphers =
      EXP, LOW, MEDIUM, aNULL, eNULL,
      SRP, PSK, kDH, ADH, AECDH, kRSA, DSS, RC4, DES, IDEA, SEED, ARIA, AESCCM8, 3DES, MD5
smtp_tls_mandatory_exclude_ciphers =
      EXP, LOW, MEDIUM, aNULL, eNULL,
      SRP, PSK, kDH, ADH, AECDH, kRSA, DSS, RC4, DES, IDEA, SEED, ARIA, AESCCM8, 3DES, MD5
smtp_tls_session_cache_database = btree:/var/lib/postfix/smtp_tls_session_cache
smtp_dns_support_level = dnssec
smtp_tls_security_level = dane