This is custom email format validation service that hold all the cases.

email_validation_service.rb

require 'resolv.rb'

class EmailValidationService

  DOMAIN_LENGTH = 255
  LOCAL_LENGTH = 64
  LOCAL_PART_SPECIAL_CHAR = /[\!\#\$\%\&\'\*\-\/\=\?\+\-\^\_\`\{\|\}\~]/

  def self.valid_email_format?(email)
    begin
      domain, local = email.reverse.split('@', 2)
    rescue
      return false
    end

    return false if local.blank?
    return false if domain.blank?
    return false if local.length > LOCAL_LENGTH
    return false if domain.length > DOMAIN_LENGTH

    local.reverse!
    domain.reverse!

    return false unless self.validate_local_part_syntax(local) && self.validate_domain_part_syntax(domain)
    return false unless self.validate_email_domain(email)
    true
  end

  def self.validate_email_domain(email)
    domain = email.to_s.downcase.match(/\@(.+)/)[1]
    Resolv::DNS.open do |dns|
      @mx = dns.getresources(domain, Resolv::DNS::Resource::IN::MX) + dns.getresources(domain, Resolv::DNS::Resource::IN::A)
    end
    @mx.size > 0 ? true : false
  end

  def self.validate_local_part_syntax(local)
    in_quoted_pair = false
    in_quoted_string = false

    (0..local.length-1).each do |i|
      ord = local[i].ord

      # accept anything if it's got a backslash before it
      if in_quoted_pair
        in_quoted_pair = false
        next
      end

      # backslash signifies the start of a quoted pair
      if ord == 92 && i < local.length - 1
        return false unless in_quoted_string # must be in quoted string per http://www.rfc-editor.org/errata_search.php?rfc=3696
        in_quoted_pair = true
        next
      end

      # double quote delimits quoted strings
      if ord == 34
        in_quoted_string = !in_quoted_string
        next
      end

      next if local[i, 1] =~ /[a-z0-9]/i
      next if local[i, 1] =~ LOCAL_PART_SPECIAL_CHAR

      # period must be followed by something
      if ord == 46
        return false if i == 0 || i == local.length - 1 # can't be first or last char
        next unless local[i+1].ord == 46 # can't be followed by a period
      end

      return false
    end

    return false if in_quoted_string # unbalanced quotes
    return true
  end

  def self.validate_domain_part_syntax(domain)
    parts = domain.downcase.split('.', -1)

    return false if parts.length <= 1 # Only one domain part

    # Empty parts (double period) or invalid chars
    return false if parts.any? {
      |part|
        part.nil? ||
        part.empty? ||
        !(part =~ /\A[[:alnum:]\-]+\Z/) ||
        part[0, 1] == '-' || part[-1, 1] == '-' # hyphen at beginning or end of part
    }

    # ipv4
    return true if parts.length == 4 && parts.all? { |part| part =~ /\A[0-9]+\Z/ && part.to_i.between?(0, 255) }
    return false if parts[-1].length < 2 || !(parts[-1] =~ /[a-z\-]/) # TLD is too short or does not contain a char or hyphen
    return true
  end

end

After research of many gems I compiled this service, light weight and easy to integrate in code.

These are some of the results, let me know if there is any case that is not working as per expectation; any missing case indication will be appreciated.

EmailValidationService.valid_email_format?('sohair@[email protected]')     => false 
EmailValidationService.valid_email_format?('sohair@[email protected]')     => false 
EmailValidationService.valid_email_format?('sohair@gmail')              => false 
EmailValidationService.valid_email_format?('sohair@gmail.')             => false 
EmailValidationService.valid_email_format?('[email protected]')            => false 
EmailValidationService.valid_email_format?('@gmail.co')                 => false 
EmailValidationService.valid_email_format?('aaa@gmail')                 => false 
EmailValidationService.valid_email_format?('[email protected]')          => false 
EmailValidationService.valid_email_format?('[email protected]')            => false 
EmailValidationService.valid_email_format?('[email protected]')             => false 
EmailValidationService.valid_email_format?('[email protected].')             => false 
EmailValidationService.valid_email_format?('[email protected]')            => false 
EmailValidationService.valid_email_format?('[email protected]')           => true 
EmailValidationService.valid_email_format?('[email protected]')            => true 
EmailValidationService.valid_email_format?('[email protected]')          => true 
EmailValidationService.valid_email_format?('[email protected]')             => true