[Jellby]

MOBI (mobi7, I guess) does not support CSS at all. It's all the work of the converter if it looks like it does, and maybe the converter is buggy, try another one (like calibre).

But anyway, why use ".maintext" at all? This is simpler:

p {
 font-size: 1em; /* I'm pretty sure this is redundant */
 margin: 0 0 0.2em 0;
 text-indent: 1.5em;
}

.space {
 margin-bottom: 1em;
}

.noindent {
 text-indent: 0;
}

And you just use <p>, <p class="noindent">, <p class="space"> or <p class="space noindent">. But it might have the same problem upon conversion.