Can I schedule a mail delivery without using GAS trigger?

Question

Gmail has the option to schedule mailings from the application.

Is this option implemented in GmailApp.sendEmail()?

I don't want to use Google Apps Script triggers, I want to use the option given by the GmailApp

Answer 1

Is this option implemented in GmailApp.sendEmail()?

ATTOW, no, please see the official documentation

Bonus: can I schedule this using Gmail API?

ATTOW, also no, see the documentation

Workaround

I understand that you don't want to use triggers, but until the feature is implemented, you can emulate scheduling logic using a trigger and PropertiesService (or a database of your choosing if you need to store larger emails). Here is a working sample of how you may achieve this (I opted to use a 1 minute trigger, but in real world it might be more pragmatic to use a higher delay between checks):

const sendScheduledEmail = () => {
    const propertyName = "emailSchedule";

    const store = PropertiesService.getUserProperties();

    const savedSchedule = store.getProperty(propertyName) || "[]";

    /** @type {emailConfig[]} */
    const parsed = JSON.parse(savedSchedule);

    if(!parsed.length) {
        console.log("Nothing to send");
        return;
    }

    const currentDT = Date.now();

    const leftToSend = parsed.filter(emailConfig => {

        const { 
            delay, 
            subject, 
            recipient, 
            body, 
            options 
        } = emailConfig;

        if (currentDT > delay) {
            GmailApp.sendEmail(recipient, subject, body, options);
            return false;
        }

        return true;
    });

    store.setProperty(propertyName, JSON.stringify(leftToSend));
};

/**
 * @typedef {object} delayedConfig
 * @property {number} delay
 * 
 * @typedef {object} commonEmailConfig
 * @property {string} body
 * @property {string} recipient
 * @property {string} subject
 * @property {GoogleAppsScript.Gmail.GmailAdvancedOptions} options
 * 
 * @typedef {commonEmailConfig & delayedConfig} emailConfig
 * 
 * @param {...emailConfig} configs 
 * @returns {boolean}
 */
const scheduleEmailSend = (...configs) => {

    try {

        const now = Date.now();

        const normalizedDelayConfigs = configs.map(config => {
            const { delay } = config;
            config.delay = now + delay;
            return config;
        });

        const checkTriggerName = "sendScheduledEmail", propertyName = "emailSchedule";

        const triggers = ScriptApp.getProjectTriggers();

        const [trigger] = triggers.filter(trig => trig.getHandlerFunction() === checkTriggerName);

        !trigger && ScriptApp.newTrigger(checkTriggerName).timeBased().everyMinutes(1).create();

        const store = PropertiesService.getUserProperties();

        const savedSchedule = store.getProperty(propertyName) || "[]";

        /** @type {emailConfig[]} */
        const parsed = JSON.parse(savedSchedule);

        parsed.push(...normalizedDelayConfigs);

        store.setProperty(propertyName, JSON.stringify(parsed));


    } catch (error) {
        console.warn(error);
        return false;
    }

    return true;
};

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Google is already aware of the feature, please join others on the already opened request - it will have a better chance of being introduced.

Answer 2

Unfortunately, the only option available of scheduling the sending of an email is by using the triggers.

What you can do instead is to file a Feature Request for the Gmail API using Google Issue Tracker here and provide the details needed regarding this.