wekan/server/notifications/email.js

// buffer each user's email text in a queue, then flush them in single email
Meteor.startup(() => {
  Notifications.subscribe('email', (user, title, description, params) => {
    // add quote to make titles easier to read in email text
    const quoteParams = _.clone(params);
    ['card', 'list', 'oldList', 'board', 'comment'].forEach(key => {
      if (quoteParams[key]) quoteParams[key] = `"${params[key]}"`;
    });
    ['timeValue', 'timeOldValue'].forEach(key => {
      quoteParams[key] = quoteParams[key] ? `${params[key]}` : '';
    });

    const lan = user.getLanguage();
    const subject = TAPi18n.__(title, params, lan); // the original function has a fault, i believe the title should be used according to original author
    const existing = user.getEmailBuffer().length > 0;
    const text = `${existing ? `<br/>\n${subject}<br/>\n` : ''}${
      params.user
    } ${TAPi18n.__(description, quoteParams, lan)}<br/>\n${params.url}`;

    user.addEmailBuffer(text);

    // unlike setTimeout(func, delay, args),
    // Meteor.setTimeout(func, delay) does not accept args :-(
    // so we pass userId with closure
    const userId = user._id;
    Meteor.setTimeout(() => {
      const user = Users.findOne(userId);

      // for each user, in the timed period, only the first call will get the cached content,
      // other calls will get nothing
      const texts = user.getEmailBuffer();
      if (texts.length === 0) return;

      // merge the cached content into single email and flush
      const html = texts.join('<br/>\n\n');
      user.clearEmailBuffer();
      try {
        Email.send({
          to: user.emails[0].address.toLowerCase(),
          from: Accounts.emailTemplates.from,
          subject,
          html,
        });
      } catch (e) {
        return;
      }
    }, process.env.EMAIL_NOTIFICATION_TIMEOUT || 30000);
  });
});