Email subject some time have weird characters

Are there any limitations to how big the subject line can be on a Nintex notification email , We are seeing in our system, if emails have subject

with characters  more than 100 characters , we are getting a malformed subject email , this is one example of the email subject ,

?utf-8?B?TmV3IFJlc2VhcmNoIHJlcXVlc3Q6ICJDbGluaWNpYW4ncyBndWlkZSB0byBwcm9zdGF0ZSBJTVJUIHBsYW4gYXNzZXNzbWVudCBhbmQgb3B0aW1pc2F0aW9uLk1hcnRpbiBKTSwgRnJhbnR6aXMgSiwgRWFkZSBULCBDaHVuZyBQLkogTWVkIEltYWdpbmcgUmFkaWF0IE9uY29sLiAyMDEwIERlYzs1NCg2KTo1NjktNzUuI

 Are they any kind of invalid character restriction to email subject in Nintex notification?

Thanks

-Raghav