I been using the cloud function for this for a while. If you are using a gmail address make sure you create an app password. It will look like \"akdo0 kd9023 930hls\". Here is my code if it helps:

const functions = require('firebase-functions');\nconst admin = require('firebase-admin');\nconst nodemailer = require('nodemailer');\n\n// Setup Email Client\nlet transporter = nodemailer.createTransport({\n  service: 'gmail',\n  auth: {\n     user: 'your-gmail-email',\n    pass: 'your-gmail-password'\n  }\n});\n\nexports.cancellationEmail = functions.region('us-central1').\n\trunWith({\n\t\tmemory: '128MB'\n  }).https.onCall(\n  (data, context) => {\n\t\tconst apptStatus = data.apptStatus;\n\t\tconst businessName = data.businessName;\n\t\tconst customerFirstName = data.customerFirstName;\n\t\tconst customerEmail = data.customerEmail;\n    // Write your code below!\nconst emailBody = {\n        htmlReceipt: `\n            <html>\n            <head>\n                <title>Appointment ${apptStatus} for ${businessName}</title>\n            </head>\n            <body>\n\n                <p>Hi, ${customerFirstName} your appointment today at ${businessName} status has been changed to ${apptStatus}. This is an automated email; please call for any further help.</p>\n\n                <p>Please do not reply to this email.</p>\n\n                <p>Best Regards,</p>\n                <p>${businessName}</p>\n            </body>\n            </html>\n        `\n    };\n\n    const mailOptions = {\n        from: `${businessName} <your-email-address>`,\n        to: customerEmail,\n        subject: `Your Appointment ${apptStatus} at ${businessName}`,\n        html: emailBody.htmlReceipt \n    };\n\n    return new Promise((resolve, reject) => {\n        transporter.sendMail(mailOptions, (error, info) => {\n            if (error) {\n                console.log('Error sending email:', error);\n                reject(error);\n            } else {\n                console.log('Email sent: %s', info.messageId);\n                resolve(info);\n            }\n        });\n    });\n    // Write your code above!\n  }\n);

Then the package.json file:

{\n  \"name\": \"functions\",\n  \"description\": \"Firebase Custom Cloud Functions\",\n  \"engines\": {\n    \"node\": \"18\"\n  },\n  \"main\": \"index.js\",\n  \"dependencies\": {\n    \"firebase-admin\": \"^11.8.0\",\n    \"firebase-functions\": \"^4.3.1\",\n    \"nodemailer\": \"^6.9.3\"\n  },\n  \"private\": true\n}

Sending Email