In openpop net when read email it's get an error "The input is not valid base64 string as it contains non-base 64 character,"

Question

I use openpop net library for delete and forward email, normal mail(without TLS protocol) contains are readable. But which mails are sent through protocol(TLS) are not readable, that give me an error.

"The input is not valid base64 string as it contains non-base 64 character, more than two padding character or an illeagal character".