Why is Android e-mail doing this? Changing text to lines of random letters.

The last few emails I have sent from my S3 have not sent properly. Instead of the text I have sent, the recipient is receiving an email full of gibberish. I have copied a line below from my sent emails to give an example.

ZSBiZXN0IHNvbHV0aW9uIHdvdWxkIGJlIHRvIGRlbW9saXNoIHRoZSBzaGVsdGVycyBhcyB0aGUg

Why is doing this and how do I fix it? I've had a look on Google and can't make sense of it.
Thanks.