[MlMt] Limit text width for received format=flowed mails

[Dato Simó]

> 	p { max-width: 30em; }
> 	div[markup="none"] div { max-width: 30em; }

That did the trick, thank you. 40em is perfect.

> I cannot guarantee that it'll work well for all emails, but it might 
> be good enough for you.

Ack, thanks.