I see several issues with your SMTP session.

First, gmail.com will be protected by SPF and DKIM and your message will likely be flagged as spam (or outright rejected) because it’s clear that you’re not sending on behalf of the real gmail.com.

Second, commands should be in all-caps. A server may accept or reject lowercase keywords.

Third, you need to leave a blank line between the mail headers and the body, so that part of your session would look like so: …

DATA
354 Go ahead
From: ...
To: ...
Subject: ...

This is the first line of text.
This is the second line.
.
250 Queued

Having said that, many servers will require an encrypted connection (SMTPS), many ISPs will block port 25 for residential customers as an anti-spam measure (so your local mail server may accept the message from your script but be unable to forward it), ESMTP should be preferred over SMTP etc.
If at all possible, you should use a full-featured mail library for this and use your ISP’s own mail server.

Doesn’t Pop_OS come with a sendmail command?

If you’re ok using Signal I use https://github.com/AsamK/signal-cli for sending myself texts through command line

Implementing a telegram or discord bot might be easier

Yes, you can. Though as another commenter mentioned, doing it like you currently attempted to, is way too much of a hassle.

That is, don’t try and negotiate your own SMTP session and content. Being POP_OS is Ubuntu based, you should be able to use the mail command from mailutils package

echo "Is this working?" | mail -s "Subject" recipient@email-address.org

Also might want to consider something like Apprise (Everything and the kitchen sink) or NTFY (Does one thing, does it well) for other types of notification methods.

Your current approach of talking raw SMTP is likely to be more hassle than is worthwhile, and since the days of permissive SMTP servers are long gone, might not work at all.

Since you appear to be using an Debian-based Linux distro, I suggest this approach:

• If you don’t specifically need exim, consider replacing it with the lightweight dma package (DragonFly Mail Agent): apt install dma
• Configure dma (or exim) to use your ISP’s SMTP server as a smart host. (Or the Gmail SMTP server if your ISP doesn’t provide one.)
• Use the /usr/sbin/sendmail command (which comes with dma or exim) to send messages from your scripting language of choice.

If you prefer to receive messages as SMS, note that most major mobile carriers maintain an email-to-sms gateway for this purpose. Some web searches will probably lead you to the one for your carrier. They usually accept email at an address like 123456789@sms-gateway.example.com

Any reason not to use something like Gotify and handle it as a notification instead?

Many cell providers have a mail to SMS gateway. Just sent email to the correct address and you’ll get SMS. As far as sending mail either with bash or with Python. That is quite possible and not hard.

Sorry I do not have the code at hand. Memory is Python standard library has SMTP capability and Bash you can just use the mail command. May have to configure mail on your Linux box too or just use a remote server.

Standard tool to send email is mailx, it is also often aliased as mail.

You’re going to have to use an external service for email perhaps connected to something like Postfix or just using the email provider’s API - something else to think of if you just need something on your phone as a message / reminder is to use KDE Connect like in this tutorial https://doronbehar.com/articles/using-kdeconnect-to-comfortably-send-sms-messages-from-the-shell/

You can fire a notification to AWS SNS and have it email, text, fire a program, anything.

FYI, I cross posted your question to three Programming.dev communities:

Shell Scripting

Python

Learn Programming

If all you need is to send a notification from a script to your phone, I’d like to suggest https://ntfy.sh/

As somebody who has been there before, it’s 100% possible to use email for notifications and have that fully scriptable, but given how extremely stringent email providers have become over the decades and still getting tougher as we speak, i would very much recommend not depending on email for this. Ideally you’d have multiple channels in case one fails for the REALLY vital stuff, but at least you should have one that is not email. There’s many:

• One of the various ways to generate push notifications that have been mentioned in the thread
• Chat messages, people have mentioned using Telegram, Signal or others, and some of those have desktop version too
• Yes, email, but not as the only way, you don’t wanna stop receiving notifications because Google just decided your IP is spamming and is bouncing all your alerts
• One of the various cloud notification things like the AWS ones mentioned (pretty sure all the cloud providers have their own version)
• Something like a monitoring service or self-hosted server (I’ve used Zabbix in the past and like it, although this is FAR more involved)
• Text messages? This will depend on services on your area, here they’re so rare i never consider this option
• Others i can’t think about right now (need more coffee)

Whatever you do have in mind that ANY of these can fail, so if there’s anything truly critical be sure to both have a way to know if one notification system is failing, and to have a plan B to receive notifications/alerts even if one is down. Depends on how critical stuff is of course, for tests you don’t need triple redundancy or whatever, but for the service the company’s income depends on you don’t wanna have a system that stopped working and you never knew.

I personally use python plus AWS SES to handle outgoing mail. Sure you have to have an AWS account, setup the SES feature, and get out of the AWS sandbox but once that’s front loaded you have a great little e-mail service.

Feel free to share the code or code repository. I’m interested in this as well