Graylog2 cannot send email alert

I have graylog2 v0.20.6 and when I try send test alert via mail I see this error:

Error! ×Unable to send dummy alert, check server log for details: API call failed GET http://@127.0.0.1:12900/streams/53eb19b722e4a0c4d93dcfe5/alerts/sendDummyAlert returned 500 Internal Server Error body: Sending the email to the following server failed : 192.168.169.10:587

Below I share my graylog2-server config file:

# If you are running more than one instances of graylog2-server you have to select one of these
# instances as master. The master will perform some periodical tasks that non-masters won't perform.
is_master = true

# The auto-generated node ID will be stored in this file and read after restarts. It is a good idea
# to use an absolute file path here if you are starting graylog2-server from init scripts or similar.
node_id_file = /etc/graylog2-server-node-id

# You MUST set a secret to secure/pepper the stored user passwords here. Use at least 64 characters.
# Generate one by using for example: pwgen -s 96
password_secret = 8OiOyemf3RBqK2SdmppNouFilSIgKjGShI6fgC72ixeCgRYtxMsfffsdfasfsdfefeffsdfsafsdfafeasZen2ycrRRzGEbuasoiN2q3Z6GrDNiQEGNfmAemNd4wc1
# the default root user is named 'admin'
# root_username = admin
# You MUST specify a hash password for the root user (which you only need to initially set up the
# system and in case you lose connectivity to your authentication backend)
# This password cannot be changed using the API or via the web interface. If you need to change it,
# modify it in this file.
# Create one by using for example: echo -n yourpassword | shasum -a 256
# and put the resulting hash value into the following line
root_password_sha2 = 5430d65ba153185053363eb0fdsfsdfdasdwrwerewr32c36cd38df30a2feaa6e2e876f5efadef32ce29

# Set plugin directory here (relative or absolute)
plugin_dir = plugin

# REST API listen URI. Must be reachable by other graylog2-server nodes if you run a cluster.
rest_listen_uri = http://127.0.0.1:12900/
# REST API transport address. Defaults to the value of rest_listen_uri. Exception: If rest_listen_uri
# is set to a wildcard IP address (0.0.0.0) the first non-loopback IPv4 system address is used.
# This will be promoted in the cluster discovery APIs and other nodes may try to connect on this
# address. (see rest_listen_uri)
rest_transport_uri = http://127.0.0.1:12900/

# Enable CORS headers for REST api. This is necessary for JS-clients accessing the server directly.
# If these are disabled, modern browsers will not be able to retrieve resources from the server.
# This is disabled by default. Uncomment the next line to enable it.
#rest_enable_cors = true

# Enable GZIP support for REST api. This compresses API responses and therefore helps to reduce
# overall round trip times. This is disabled by default. Uncomment the next line to enable it.
#rest_enable_gzip = true

# Embedded elasticsearch configuration file
# pay attention to the working directory of the server, maybe use an absolute path here
#elasticsearch_config_file = /etc/graylog2-elasticsearch.yml

elasticsearch_max_docs_per_index = 20000000

# How many indices do you want to keep?
# elasticsearch_max_number_of_indices*elasticsearch_max_docs_per_index=total number of messages in your setup
elasticsearch_max_number_of_indices = 20

# Decide what happens with the oldest indices when the maximum number of indices is reached.
# The following strategies are availble:
#   - delete # Deletes the index completely (Default)
#   - close # Closes the index and hides it from the system. Can be re-opened later.
retention_strategy = close

# How many ElasticSearch shards and replicas should be used per index? Note that this only applies to newly created indices.
elasticsearch_shards = 1
elasticsearch_replicas = 0

elasticsearch_index_prefix = graylog2

# Do you want to allow searches with leading wildcards? This can be extremely resource hungry and should only
# be enabled with care. See also: http://support.torch.sh/help/kb/graylog2-web-interface/the-search-bar-explained
allow_leading_wildcard_searches = false

# Do you want to allow searches to be highlighted? Depending on the size of your messages this can be memory hungry and
# should only be enabled after making sure your elasticsearch cluster has enough memory.
allow_highlighting = false

# settings to be passed to elasticsearch's client (overriding those in the provided elasticsearch_config_file)
# all these
# this must be the same as for your elasticsearch cluster
#elasticsearch_cluster_name = graylog2

# you could also leave this out, but makes it easier to identify the graylog2 client instance
#elasticsearch_node_name = graylog2-server

# we don't want the graylog2 server to store any data, or be master node
#elasticsearch_node_master = false
#elasticsearch_node_data = false

# use a different port if you run multiple elasticsearch nodes on one machine
#elasticsearch_transport_tcp_port = 9350
# we don't need to run the embedded HTTP server here
#elasticsearch_http_enabled = false

elasticsearch_discovery_zen_ping_multicast_enabled = false
elasticsearch_discovery_zen_ping_unicast_hosts = 127.0.0.1:9300

# the following settings allow to change the bind addresses for the elasticsearch client in graylog2
# these settings are empty by default, letting elasticsearch choose automatically,
# override them here or in the 'elasticsearch_config_file' if you need to bind to a special address
# refer to http://www.elasticsearch.org/guide/en/elasticsearch/reference/0.90/modules-network.html for special values here
# elasticsearch_network_host =
# elasticsearch_network_bind_host =
# elasticsearch_network_publish_host =

# Analyzer (tokenizer) to use for message and full_message field. The "standard" filter usually is a good idea.
# All supported analyzers are: standard, simple, whitespace, stop, keyword, pattern, language, snowball, custom
# ElasticSearch documentation: http://www.elasticsearch.org/guide/reference/index-modules/analysis/
# Note that this setting only takes effect on newly created indices.
elasticsearch_analyzer = standard

# Batch size for the ElasticSearch output. This is the maximum (!) number of messages the ElasticSearch output
# module will get at once and write to ElasticSearch in a batch call. If the configured batch size has not been
# reached within output_flush_interval seconds, everything that is available will be flushed at once. Remember
# that every outputbuffer processor manages its own batch and performs its own batch write calls.
# ("outputbuffer_processors" variable)
output_batch_size = 25

# Flush interval (in seconds) for the ElasticSearch output. This is the maximum amount of time between two
# batches of messages written to ElasticSearch. It is only effective at all if your minimum number of messages
# for this time period is less than output_batch_size * outputbuffer_processors.
output_flush_interval = 1

# The number of parallel running processors.
# Raise this number if your buffers are filling up.
processbuffer_processors = 5
outputbuffer_processors = 3

# Wait strategy describing how buffer processors wait on a cursor sequence. (default: sleeping)
# Possible types:
#  - yielding
#     Compromise between performance and CPU usage.
#  - sleeping
#     Compromise between performance and CPU usage. Latency spikes can occur after quiet periods.
#  - blocking
#     High throughput, low latency, higher CPU usage.
#  - busy_spinning
#     Avoids syscalls which could introduce latency jitter. Best when threads can be bound to specific CPU cores.
processor_wait_strategy = blocking

# Size of internal ring buffers. Raise this if raising outputbuffer_processors does not help anymore.
# For optimum performance your LogMessage objects in the ring buffer should fit in your CPU L3 cache.
# Start server with --statistics flag to see buffer utilization.
# Must be a power of 2. (512, 1024, 2048, ...)
ring_size = 1024

# EXPERIMENTAL: Dead Letters
# Every failed indexing attempt is logged by default and made visible in the web-interface. You can enable
# the experimental dead letters feature to write every message that was not successfully indexed into the
# MongoDB "dead_letters" collection to make sure that you never lose a message. The actual writing of dead
# letter should work fine already but it is not heavily tested yet and will get more features in future
# releases.
dead_letters_enabled = false

# How many seconds to wait between marking node as DEAD for possible load balancers and starting the actual
# shutdown process. Set to 0 if you have no status checking load balancers in front.
lb_recognition_period_seconds = 3

# MongoDB Configuration
mongodb_useauth = false
#mongodb_user = grayloguser
#mongodb_password = 123
mongodb_host = 127.0.0.1
#mongodb_replica_set = localhost:27017,localhost:27018,localhost:27019
mongodb_database = graylog2
mongodb_port = 27017

# Raise this according to the maximum connections your MongoDB server can handle if you encounter MongoDB connection problems.
mongodb_max_connections = 100

# Number of threads allowed to be blocked by MongoDB connections multiplier. Default: 5
# If mongodb_max_connections is 100, and mongodb_threads_allowed_to_block_multiplier is 5, then 500 threads can block. More than that and an exception will be thrown.
# http://api.mongodb.org/java/current/com/mongodb/MongoOptions.html#threadsAllowedToBlockForConnectionMultiplier
mongodb_threads_allowed_to_block_multiplier = 5

# Drools Rule File (Use to rewrite incoming log messages)
# See: http://support.torch.sh/help/kb/graylog2-server/custom-message- rewritingprocessing
# rules_file = /etc/graylog2.drl

# Email transport
transport_email_enabled = true
transport_email_protocol = smtp
transport_email_hostname = 192.168.159.10
transport_email_port = 587
transport_email_use_auth = false
transport_email_use_tls = false
transport_email_auth_username = graylog2@xyz.com
transport_email_auth_password = W6eteqAqeye3
transport_email_subject_prefix = [graylog2]
transport_email_from_email = graylog2@xyz.com
transport_email_from_name = Graylog2
transport_email_web_interface_url = http://192.168.178.133:3000

# Specify and uncomment this if you want to include links to the stream in your stream alert mails.
# This should define the fully qualified base url to your web interface exactly the same way as it is accessed by your users.
#
#transport_email_web_interface_url = http://192.168.178.133:3000
# HTTP proxy for outgoing HTTP calls
#http_proxy_uri = 

Anybody have some idea what's wrong, and why graylog2 cannot send email notification.

Regards
Mick