Redmine

Issue:
After reboot my redmine PC, notice that all the notication email are not send out. Checked on Error log, saw the `block in deliver!' error. Need help to resolve this issue. Thank you.

Error Log:

App 2195 output: #<Thread:0x00007f279795e1b8@/data/redmine/redmine-3.4.4/config/initializers/10-patches.rb:111 run> terminated with exception (report_on_exception is true):
App 2195 output: /usr/share/rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/net/smtp.rb:539:in `initialize': getaddrinfo: Temporary failure in name resolution (SocketError)
App 2195 output:     from /usr/share/rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/net/smtp.rb:539:in `open'
App 2195 output:     from /usr/share/rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/net/smtp.rb:539:in `tcp_socket'
App 2195 output:     from /usr/share/rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/net/smtp.rb:549:in `block in do_start'
App 2195 output:     from /usr/share/rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/timeout.rb:93:in `block in timeout'
App 2195 output:     from /usr/share/rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/timeout.rb:103:in `timeout'
App 2195 output:     from /usr/share/rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/net/smtp.rb:548:in `do_start'
App 2195 output:     from /usr/share/rvm/rubies/ruby-2.5.0/lib/ruby/2.5.0/net/smtp.rb:518:in `start'
App 2195 output:     from /usr/share/rvm/gems/ruby-2.5.0/gems/mail-2.6.6/lib/mail/network/delivery_methods/smtp.rb:111:in `deliver!'
App 2195 output:     from /data/redmine/redmine-3.4.4/config/initializers/10-patches.rb:112:in `block in deliver!'

Redmine information
Environment:
Redmine version 3.4.4.stable
Ruby version 2.5.0-p0 (2017-12-25) [x86_64-linux]
Rails version 4.2.8
Environment production
Database adapter Mysql2
SCM:
Git 2.7.4
Filesystem
Redmine plugins:
redmine_mentions 0.1.1
redmine_wiki_extensions 0.8.2
redmine_wiki_lists 0.0.9
wiki_issue_fields 0.5.2