Out of curiosity I tested this with a map with file size 131 MB. A sort of new message dialog opened in Thunderbird. "Sort of" because there were no fields for sender nor any receivers, this smells like a bug in Thunderbird. The map was attached to it but it was only 386 kB in size!! I saved the attachment and opened it in Xmind, got an error message saying that file is corrupted. Okey, I know that attachments over 10 MB will not get through email servers... Anyway, I wonder what the heck happened to attachment in the process, either Xmind or Thunderbird messed up it totally.
Using this as pons asinorum, I think you are better served if you share your map using some file share service (Xmind Share, Dropbox, iCloud, Google drive, OneDrive...). Attachments in emails are nowdays considered evil in many organisations. I agree.