Mailx es un cliente que nos permite desde la línea de comandos de linux enviar correos usando servidor externo

Instalación

yum install -y mailx
ln -s /bin/mailx /bin/email
nano /etc/mail.rc
set smtp=smtp://smtp_server.com.mx:587
set smtp-use-starttls
set ssl-verify=ignore
set smtp-auth=login
set smtp-auth-user=correo_relay@com.mx
set smtp-auth-password=contraseña
set nss-config-dir=/etc/pki/nssdb/

Ahora ya se puede usar mail para mandar correo via servidor remoto, a continuación de un ejemplo en de job de crontab

!/bin/sh
 aide check - SShameer
 DATE=date +%Y-%m-%d
 echo $DATE
 REPORT="Aide-"$DATE.txt
 echo $REPORT
 echo "System check !! date" > /tmp/$REPORT
 aide --check > /tmp/aidecheck.txt
 cat /tmp/aidecheck.txt|/bin/grep -v failed >> /tmp/$REPORT
 echo "" >> /tmp/$REPORT tail -20 /tmp/aidecheck.txt >> /tmp/$REPORT echo "**DONE" >> /tmp/$REPORT
 mail -s "$REPORT date" cesar.vazquez@kpt.com.mx< /tmp/$REPORT

Para hacer una prueba y ver que se envian correos realizar este test

echo "Prueba" | /bin/mail -v -q -s "subject" -r usuario_correo@com.mx -S smtp-auth=login -S smtp=smtps://smtp_server.com.mx:587 -S smtp-auth-user="correo_que_tiene_relay@com.mx" -S smtp-auth-password="password" correo_destino@com.mx

Si al hacer la prueba da este error, realizar el workaroud de abajo

Error in certificate: Peer's certificate issuer is not recognized.

openssl s_client -showcerts -connect mail2.kpt.com.mx:465 </dev/null

Esto descargara en pantalla el certificado publico del servidor de correo

CONNECTED(00000003)
 depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3
 verify return:1
 depth=1 C = US, O = Let's Encrypt, CN = R3
 verify return:1
 depth=0 CN = *.kpt.com.mx
 verify return:1
 Certificate chain
  0 s:/CN=*.kpt.com.mx
    i:/C=US/O=Let's Encrypt/CN=R3
 -----BEGIN CERTIFICATE-----
 MIIFizCCBHOgAwIBAgISAy9d64R4nxw6PDFSAZ8mpXTCMA0GCSqGSIb3DQEBCwUA
 MDIxCzAJBgNVBAYTAlVTMRYwFAYDVQQKEw1MZXQncyBFbmNyeXB0MQswCQYDVQQD
 EwJSMzAeFw0yMTA0MTcwNTIxMjdaFw0yMTA3MTYwNTIxMjdaMBcxFTATBgNVBAMM
 DCoua3B0LmNvbS5teDCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBALv2
 FMg0U+bxwZAJ60wG08r4L+qtFNTjX+VM7llIGICvI6SzdY5tpSTAJ2CPCsEcj0Km
 yUb0oy5UA3sguKcX6wv0Sjw+G5tScJVQQWv3W2S0zTMC1AUkm3dotWIPAYo66I07
 mSs1qvs8MqwRcskNKmw7BJkqJamhNAKgN0M/SMO0zzohuQXRy7dZkyO57K17p3BV
 u3pCdUsiTjje+ydBDn2LUA2mFlhOTDmF39BribOoPqC35pfrRGPYQKn/OpmPdWo2
 9HRUyxs3zeci8UcMqdD28El2ZjrT3RXm7de3jsOOV6fcakeg5WiHWREzwYxEjJST
 SH5bHpzIpJW5wO0n8zUCAwEAAaOCArQwggKwMA4GA1UdDwEB/wQEAwIFoDAdBgNV
 HSUEFjAUBggrBgEFBQcDAQYIKwYBBQUHAwIwDAYDVR0TAQH/BAIwADAdBgNVHQ4E
 FgQUUxUrR0Jx1fYX54p6xbs+MiuN94wwHwYDVR0jBBgwFoAUFC6zF7dYVsuuUAlA
 5h+vnYsUwsYwVQYIKwYBBQUHAQEESTBHMCEGCCsGAQUFBzABhhVodHRwOi8vcjMu
 by5sZW5jci5vcmcwIgYIKwYBBQUHMAKGFmh0dHA6Ly9yMy5pLmxlbmNyLm9yZy8w
 gYUGA1UdEQR+MHyCFSouYW1vcmNpdG9jb3Jhem9uLm5ldIIQKi5hdXN0cmlhLmNv
 bS5teIIMKi5hdXN0cmlhLm14ghEqLmNsdWJtYW51LmNvbS5teIINKi5rOXB0LmNv
 bS5teIIMKi5rcHQuY29tLm14ggoqLm5heWFhLm14ggdrOXB0Lm14MEwGA1UdIARF
 MEMwCAYGZ4EMAQIBMDcGCysGAQQBgt8TAQEBMCgwJgYIKwYBBQUHAgEWGmh0dHA6
 Ly9jcHMubGV0c2VuY3J5cHQub3JnMIIBAgYKKwYBBAHWeQIEAgSB8wSB8ADuAHUA
 RJRlLrDuzq/EQAfYqP4owNrmgr7YyzG1P9MzlrW2gagAAAF43n3FYgAABAMARjBE
 AiACRbRZhL6HEVXN09YBlIzNL2jnP2B0VXybZit3y7+UKwIgCZ2furRY+1e9d6gY
 15ph1IRrP02C+a4pdIL4tzr9kCIAdQB9PvL4j/+IVWgkwsDKnlKJeSvFDngJfy5q
 l2iZfiLw1wAAAXjefcWdAAAEAwBGMEQCIDDF0FA7MU66IISrmkt1lvqc8yBSr+Ss
 wojU4I+8WIwlAiBmAPpqaNWTawmNmF7KhYF7tnH2tXwXS0fBNfDZFj6ZNTANBgkq
 hkiG9w0BAQsFAAOCAQEAlRfJVE3a8JNFJemVhtrQhu4KjoFAgQreWqVDW0cfK0fC
 szoXmaKPzW5eB7jU+zAxfGyCDWtEQ1T9XRf0SKKCMX9XB7R4Il3/MmCRSnGek3AG
 8mA4YZVMbeQtZ+dVNSug1wspkfT1HBKqM1OK2xgErqgS1o6sGx11LRUyElndWWP9
 bym26Rsy5xcp3nDT4BuwfusUB7i5LJnhFfhn51fAQy5+FipZAtZjhIvAiUc3cVtM
 zDCbZjFGCclKTws1gLuw0tSN1IXhmeXqup1cwdVSiiD9eJW6qs7Fg9ooDhdINIiI
 I4e0/rSFbd0NTguAw6O8zjhUMSjlJ1uqTquNqEI3Mg==
 -----END CERTIFICATE-----
  1 s:/C=US/O=Let's Encrypt/CN=R3
    i:/O=Digital Signature Trust Co./CN=DST Root CA X3
 -----BEGIN CERTIFICATE-----
 MIIEZTCCA02gAwIBAgIQQAF1BIMUpMghjISpDBbN3zANBgkqhkiG9w0BAQsFADA/
 MSQwIgYDVQQKExtEaWdpdGFsIFNpZ25hdHVyZSBUcnVzdCBDby4xFzAVBgNVBAMT
 DkRTVCBSb290IENBIFgzMB4XDTIwMTAwNzE5MjE0MFoXDTIxMDkyOTE5MjE0MFow
 MjELMAkGA1UEBhMCVVMxFjAUBgNVBAoTDUxldCdzIEVuY3J5cHQxCzAJBgNVBAMT
 AlIzMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuwIVKMz2oJTTDxLs
 jVWSw/iC8ZmmekKIp10mqrUrucVMsa+Oa/l1yKPXD0eUFFU1V4yeqKI5GfWCPEKp
 Tm71O8Mu243AsFzzWTjn7c9p8FoLG77AlCQlh/o3cbMT5xys4Zvv2+Q7RVJFlqnB
 U840yFLuta7tj95gcOKlVKu2bQ6XpUA0ayvTvGbrZjR8+muLj1cpmfgwF126cm/7
 gcWt0oZYPRfH5wm78Sv3htzB2nFd1EbjzK0lwYi8YGd1ZrPxGPeiXOZT/zqItkel
 /xMY6pgJdz+dU/nPAeX1pnAXFK9jpP+Zs5Od3FOnBv5IhR2haa4ldbsTzFID9e1R
 oYvbFQIDAQABo4IBaDCCAWQwEgYDVR0TAQH/BAgwBgEB/wIBADAOBgNVHQ8BAf8E
 BAMCAYYwSwYIKwYBBQUHAQEEPzA9MDsGCCsGAQUFBzAChi9odHRwOi8vYXBwcy5p
 ZGVudHJ1c3QuY29tL3Jvb3RzL2RzdHJvb3RjYXgzLnA3YzAfBgNVHSMEGDAWgBTE
 p7Gkeyxx+tvhS5B1/8QVYIWJEDBUBgNVHSAETTBLMAgGBmeBDAECATA/BgsrBgEE
 AYLfEwEBATAwMC4GCCsGAQUFBwIBFiJodHRwOi8vY3BzLnJvb3QteDEubGV0c2Vu
 Y3J5cHQub3JnMDwGA1UdHwQ1MDMwMaAvoC2GK2h0dHA6Ly9jcmwuaWRlbnRydXN0
 LmNvbS9EU1RST09UQ0FYM0NSTC5jcmwwHQYDVR0OBBYEFBQusxe3WFbLrlAJQOYf
 r52LFMLGMB0GA1UdJQQWMBQGCCsGAQUFBwMBBggrBgEFBQcDAjANBgkqhkiG9w0B
 AQsFAAOCAQEA2UzgyfWEiDcx27sT4rP8i2tiEmxYt0l+PAK3qB8oYevO4C5z70kH
 ejWEHx2taPDY/laBL21/WKZuNTYQHHPD5b1tXgHXbnL7KqC401dk5VvCadTQsvd8
 S8MXjohyc9z9/G2948kLjmE6Flh9dDYrVYA9x2O+hEPGOaEOa1eePynBgPayvUfL
 qjBstzLhWVQLGAkXXmNs+5ZnPBxzDJOLxhF2JIbeQAcH5H0tZrUlo5ZYyOqA7s9p
 O5b85o3AM/OJ+CktFBQtfvBhcJVd9wvlwPsk+uyOy2HI7mNxKKgsBTt375teA2Tw
 UdHkhVNcsAKX1H7GNNLOEADksd86wuoXvg==
 -----END CERTIFICATE-----
 Server certificate
 subject=/CN=*.kpt.com.mx
 issuer=/C=US/O=Let's Encrypt/CN=R3
 No client certificate CA names sent
 Peer signing digest: SHA512
 Server Temp Key: ECDH, P-256, 256 bits
 SSL handshake has read 3214 bytes and written 415 bytes
 New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
 Server public key is 2048 bit
 Secure Renegotiation IS supported
 Compression: NONE
 Expansion: NONE
 No ALPN negotiated
 SSL-Session:
     Protocol  : TLSv1.2
     Cipher    : ECDHE-RSA-AES256-GCM-SHA384
     Session-ID: D7D92A3D79715C189014DA3BBF1CD1C04B5269699A7C0980F8B7729EB07F74D8
     Session-ID-ctx: 
     Master-Key: 1E7113B7C479C31E6F20C0F47EBADDE020ACBC2F967D70F0E0F6D8912FE8548D8227769113820D4A062D685DD7FDA499
     Key-Arg   : None
     Krb5 Principal: None
     PSK identity: None
     PSK identity hint: None
     TLS session ticket lifetime hint: 7200 (seconds)
     TLS session ticket:
     0000 - d3 eb 35 b9 f5 f6 da 22-1a 9a 5e 24 e9 b2 73 26   ..5…."..^$..s&
     0010 - a2 ea d0 52 83 af f9 a8-e0 3f eb 50 68 aa 59 fd   …R…..?.Ph.Y.
     0020 - 8d c0 a9 71 b0 0d 30 b7-6e 37 cb 90 3f 50 2c 77   …q..0.n7..?P,w
     0030 - c4 81 18 b9 a2 a6 9d b6-c4 65 24 a8 e5 af f6 26   ………e$….&
     0040 - b6 00 40 15 10 b7 b7 a3-1c 56 58 29 9c 3a 2f 88   ..@……VX).:/.
     0050 - 50 0c 33 89 4c 60 ea ee-4a a0 1f 2d b9 57 7b 44   P.3.L`..J..-.W{D
     0060 - 40 75 06 26 4b b1 c5 49-a4 91 cc 72 d1 c4 4e aa   @u.&K..I…r..N.
     0070 - 66 96 66 5c 6d 10 4d 87-9d 6c 57 f9 0a a2 34 f6   f.f\m.M..lW…4.
     0080 - 4c 3b e4 af 6e b0 6d 27-b0 09 88 8d 44 fd 84 b0   L;..n.m'….D…
     0090 - dd 76 cb 1b 8e aa f3 54-14 bb 57 33 71 c3 21 04   .v…..T..W3q.!.
 Start Time: 1619320113 Timeout   : 300 (sec)
     Verify return code: 0 (ok)
 DONE

Se obtienen los datos de la entidad certificadora, en este caso es el certificado 1

O=Let's Encrypt

En un archivo por separado se copia el certificado incluyendo el BEGIN CERTIFICATE y el END CERTIFICATE

nano /lets.pem
 -----BEGIN CERTIFICATE-----
 MIIEZTCCA02gAwIBAgIQQAF1BIMUpMghjISpDBbN3zANBgkqhkiG9w0BAQsFADA/
 MSQwIgYDVQQKExtEaWdpdGFsIFNpZ25hdHVyZSBUcnVzdCBDby4xFzAVBgNVBAMT
 DkRTVCBSb290IENBIFgzMB4XDTIwMTAwNzE5MjE0MFoXDTIxMDkyOTE5MjE0MFow
 MjELMAkGA1UEBhMCVVMxFjAUBgNVBAoTDUxldCdzIEVuY3J5cHQxCzAJBgNVBAMT
 AlIzMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuwIVKMz2oJTTDxLs
 jVWSw/iC8ZmmekKIp10mqrUrucVMsa+Oa/l1yKPXD0eUFFU1V4yeqKI5GfWCPEKp
 Tm71O8Mu243AsFzzWTjn7c9p8FoLG77AlCQlh/o3cbMT5xys4Zvv2+Q7RVJFlqnB
 U840yFLuta7tj95gcOKlVKu2bQ6XpUA0ayvTvGbrZjR8+muLj1cpmfgwF126cm/7
 gcWt0oZYPRfH5wm78Sv3htzB2nFd1EbjzK0lwYi8YGd1ZrPxGPeiXOZT/zqItkel
 /xMY6pgJdz+dU/nPAeX1pnAXFK9jpP+Zs5Od3FOnBv5IhR2haa4ldbsTzFID9e1R
 oYvbFQIDAQABo4IBaDCCAWQwEgYDVR0TAQH/BAgwBgEB/wIBADAOBgNVHQ8BAf8E
 BAMCAYYwSwYIKwYBBQUHAQEEPzA9MDsGCCsGAQUFBzAChi9odHRwOi8vYXBwcy5p
 ZGVudHJ1c3QuY29tL3Jvb3RzL2RzdHJvb3RjYXgzLnA3YzAfBgNVHSMEGDAWgBTE
 p7Gkeyxx+tvhS5B1/8QVYIWJEDBUBgNVHSAETTBLMAgGBmeBDAECATA/BgsrBgEE
 AYLfEwEBATAwMC4GCCsGAQUFBwIBFiJodHRwOi8vY3BzLnJvb3QteDEubGV0c2Vu
 Y3J5cHQub3JnMDwGA1UdHwQ1MDMwMaAvoC2GK2h0dHA6Ly9jcmwuaWRlbnRydXN0
 LmNvbS9EU1RST09UQ0FYM0NSTC5jcmwwHQYDVR0OBBYEFBQusxe3WFbLrlAJQOYf
 r52LFMLGMB0GA1UdJQQWMBQGCCsGAQUFBwMBBggrBgEFBQcDAjANBgkqhkiG9w0B
 AQsFAAOCAQEA2UzgyfWEiDcx27sT4rP8i2tiEmxYt0l+PAK3qB8oYevO4C5z70kH
 ejWEHx2taPDY/laBL21/WKZuNTYQHHPD5b1tXgHXbnL7KqC401dk5VvCadTQsvd8
 S8MXjohyc9z9/G2948kLjmE6Flh9dDYrVYA9x2O+hEPGOaEOa1eePynBgPayvUfL
 qjBstzLhWVQLGAkXXmNs+5ZnPBxzDJOLxhF2JIbeQAcH5H0tZrUlo5ZYyOqA7s9p
 O5b85o3AM/OJ+CktFBQtfvBhcJVd9wvlwPsk+uyOy2HI7mNxKKgsBTt375teA2Tw
 UdHkhVNcsAKX1H7GNNLOEADksd86wuoXvg==
 -----END CERTIFICATE-----

Se agrega ahora el certificado a la base de datos

certutil -A -t "C,," -n "Let's Encrypt" -d /etc/pki/nssdb/ -i lets.pem

Se valida que se haya agregado el certificado

certutil -L  -d /etc/pki/nssdb/

Certificate Nickname                                         Trust Attributes
                                                              SSL,S/MIME,JAR/XPI
 Let's Encrypt                                                C,,

Realizar la prueba nuevamente de correo y el correo debe ser exitoso ahora