I've send someone a pretty long MMS (just text) but he wasn't able to receive. Is there a way to retransmit via EMail. E.g. copy the whole text and paste it into an email instead?
I think so.
Find the message you sent in the "Messaging" app.
Long-press on it.
Choose "Copy message text".
Now run the mail app, compose an email, and in the body of the message, long-press, and choose "Paste" from the pop-up menu.