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

I have to use <br> instead of <br>

I suspect those two tags were different when you wrote them, and Lemmy mangled them like it did in my comments. Can you describe the tags you originally wrote, to avoid the mangling? I’m curious about what exactly worked for you.

Does Qt Quick expose Qt’s markdown support? That could be another way to populate a rich text control, perhaps letting you work around the problem.

Just a guess, but have you tried <p></p> ?

Or a single p tag with or without a slash after the p? (Lemmy keeps mangling this one when I write it explicitly.)

https://doc.qt.io/qt-6/richtext-html-subset.html