When I write emails I avoid starting lines with "From ". This is a habit I got into when it was common for mail clients to use the "mboxo" format. Those programs store their emails in a big text file that looks like:

From ...
From: a@example.com
To: b@example.com
Subject: ...

Message body

From ...
From: a@example.com
To: b@example.com
Subject: ...

Message body

Each time there's a line starting with the five letters "From ", that starts a new message. If you write a message starting with From then mboxo-using mail clients will "munge" it, changing it to ">From ". So we might have:

From ...
From: a@example.com
To: b@example.com
Subject: ...

>From what you said yesterday I wasn't
thinking you would come to dinner today?

From ...
From: a@example.com
To: b@example.com
Subject: ...

Message body

Unfortunately, these clients don't also change lines starting with ">From " to ">>From " [1] which means this change isn't reversible. So someone reading the message in their mail client will see the munged version:

>From what you said yesterday I wasn't
thinking you would come to dinner today?

To avoid this, for years I've been trying not to start paragraphs with "From ", though there was always some risk my email would end up with a line starting that way due to being automatically word-wrapped.

At this point, however, I don't think the contortions are worth it, and I'm planning to stop worrying about it. People still using mboxo can see unsightly from-munges, or they can move to something using a better file format.

[1] Though some other mail clients do; we say they're using mboxrd.