I completely clear the contents of the text box before entering the letter, so there shouldn't be any leftover end-of-line characters lurking in there.

For example, our URL to our online payment portal that I put in the cover sheet of all invoices we send out is https://sweetlife.com/members/maintenance-fees/. I paste it in right out of the address bar of my browser, and there are no breaks in it. When I paste it into the cover sheet, it is clearly one unbroken line of text. However, because there are words before it, a line break is added partway through the URL.

So instead of:
Did you know? You can pay bills online at https://sweetlife.com/members/maintenance-fees/
It comes out as:
Did you know? You can pay bills online at https://sweetlife.com/members/maintenance
-fees/
The URL is broken, and won't take clickers to the appropriate page.

I've fixed this particular instance by placing the full URL on the next line alone, which can look odd, but is an improvement over a broken URL.

Whenever I use an email cover sheet through Adagio, there seems to be a character limit at which it inserts the line break so emails come out as a neat little column of text. Only, when viewed on a screen that's too narrow (like a mobile device), there isn't room for the width of the line Adagio likes, and I end up with line breaks in weird places.

For example, the start of the letter I was trying to send had line breaks like this when viewed on my mobile device:

Dear members,
This year has been quite a challenge and continues to be so. The
global
COVID-19 pandemic has definitely impacted our resort and its
ability to
operate in its fullest capacity.

When viewed with enough space, it looks like this:

Dear members,
This year has been quite a challenge and continues to be so. The global
COVID-19 pandemic has definitely impacted our resort and its ability to
operate in its fullest capacity.

I'd rather just have the text wrap naturally, so it can adapt to any screen size:
Dear members,
This year has been quite a challenge and continues to be so. The global COVID-19 pandemic has definitely impacted our resort and its ability to operate in its fullest capacity.

I'm not sure where else these line breaks could be coming from if not Adagio. If there's something I'm missing, PLEASE let me know!