Question

e.g.: comma seperated in a single textfield: [e-mail], mail2@someotherdomain, …

Answer 1

You can use the TMail::Address module to validate an email as shown here. Custom validations can be added with the validate method.

validate :check_email_addresses

def check_email_addresses
  email_addresses.split(/,\s*/).each do |email|
    TMail::Address.parse(email)
  end
rescue TMail::SyntaxError
  errors.add(:email_addresses, "are not valid")
end

---

Update: The TMail::Address module seems to be too lax on what is considered a valid email address (see comments below) so instead you can use a regular expression.

validate :check_email_addresses

def check_email_addresses
  email_addresses.split(/,\s*/).each do |email| 
    unless email =~ /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i
      errors.add(:email_addresses, "are invalid due to #{email}")
    end
  end
end

There are a variety of regular expression solutions for validating an email address. See this page for details.