In my campaign, I send an HTML version as well as a plain text version of each email. I do a lot of testing and use inbox previews to check how the HTML version is working. And then I decided to do a few tests to make sure the plain text version looks right.

In some clients, ones that have an explicit "View Text" function like Thunderbird, the plain text version would look fine.

But, I noticed that in some clients like Hotmail and AOL and some others, where the only way to view the plain text is to use something like "view Source", the plain text version would show up with a lot of odd characters. For example, anywhere there was an = sign, it would show up instead as =3D. Quote marks and apostrophes and some other characters would also show up as different codes. This was from using the "HTML to Plain" function in the ESP to create the text version from the HTML version. If I went through and manually re-typed in these different characters, then they would work and not come out as code.

However, the problem is that the = signs are in many of the links, including the auto-generated tracking form of the links and the unsubscribe and change contact information links at the bottom, which I have no control over. When the = sign comes out as =3D in these places, the links, if copied and pasted, no longer work.

I figured out that the problem is that the provider uses quoted-printable as the content transfer encoding type on these plain text messages, which mean that for certain characters it shows their hex code instead of just the character. I've let the provider know about this and how using this form of encoding is causing these characters to come out as code and breaking links, including even their own unsubscribe links. And I've let them know that most providers, on the plain text, use 7bit or Base64 as the content transfer encoding type, which doesn't have these problems.

Perhaps they'll fix this, perhaps not.

In the meantime, my question is how much should I care about this and where should I really worry about making sure the plain text version and its links work?

Which clients should I really care about testing the plain text version in? Should it bother me that people will get a text version with useless broken links if they use "View Source" in something like Hotmail or AOL where there is no explicit "View Text" function? Or is that very rare that anyone would view the plain text on those providers and should I only really care about how the text version looks in email clients where there is a specific function or setting for it like Thunderbird (and which clients would these be)?

I'd really appreciate any advice.