Given that this is authored by the CTO of Sparrow, one can safely assume that this is the library that’s at the core of Sparrow.
Here’s him talking about the construction of the library:
I found no decent email applications on this OS and becamed obsessed with building one that would satisfy my needs. I started contributing to an email application project named Claws-Mail. I quickly wanted to add IMAP support to it but the protocol was not widespread yet. When I realized that I would have to re-architect the whole project to do it, Libetpan was born. Libetpan is an Open Source framework that I initiated in 2001 that implements all email protocols (mainly POP, IMAP and SMTP) in C language.