Provides the option to parameterize mailers in order to share instance variable setup, processing, and common headers.

Consider this example that does not use parameterization:

class InvitationsMailer < ApplicationMailer
  def account_invitation(inviter, invitee)
    @account = inviter.account
    @inviter = inviter
    @invitee = invitee

    subject = "#{@inviter.name} invited you to their Basecamp (#{@account.name})"

    mail \
      subject:   subject,
      to:        invitee.email_address,
      from:      common_address(inviter),
      reply_to:  inviter.email_address_with_name
  end

  def project_invitation(project, inviter, invitee)
    @account = inviter.account
    @project = project
    @inviter = inviter
    @invitee = invitee
    @summarizer = ProjectInvitationSummarizer.new(@project.bucket)

    subject = "#{@inviter.name.familiar} added you to a project in Basecamp (#{@account.name})"

    mail \
      subject:   subject,
      to:        invitee.email_address,
      from:      common_address(inviter),
      reply_to:  inviter.email_address_with_name
  end

  def bulk_project_invitation(projects, inviter, invitee)
    @account  = inviter.account
    @projects = projects.sort_by(&:name)
    @inviter  = inviter
    @invitee  = invitee

    subject = "#{@inviter.name.familiar} added you to some new stuff in Basecamp (#{@account.name})"

    mail \
      subject:   subject,
      to:        invitee.email_address,
      from:      common_address(inviter),
      reply_to:  inviter.email_address_with_name
  end
end

InvitationsMailer.account_invitation(person_a, person_b).deliver_later

Using parameterized mailers, this can be rewritten as:

class InvitationsMailer < ApplicationMailer
  before_action { @inviter, @invitee = params[:inviter], params[:invitee] }
  before_action { @account = params[:inviter].account }

  default to:       -> { @invitee.email_address },
          from:     -> { common_address(@inviter) },
          reply_to: -> { @inviter.email_address_with_name }

  def account_invitation
    mail subject: "#{@inviter.name} invited you to their Basecamp (#{@account.name})"
  end

  def project_invitation
    @project    = params[:project]
    @summarizer = ProjectInvitationSummarizer.new(@project.bucket)

    mail subject: "#{@inviter.name.familiar} added you to a project in Basecamp (#{@account.name})"
  end

  def bulk_project_invitation
    @projects = params[:projects].sort_by(&:name)

    mail subject: "#{@inviter.name.familiar} added you to some new stuff in Basecamp (#{@account.name})"
  end
end

InvitationsMailer.with(inviter: person_a, invitee: person_b).account_invitation.deliver_later