[sylpheed:36962] SSL cert failure, "Always accept" doesn't

Hi, Sylpheed users and devs.

I've been using Sylpheed for years, and am running into an odd issue.
I run my own mailserver, and after years of using a self-signed SSL
certificate, I now use one from Let's Encrypt.  Ever since I've done that,
when I start Sylpheed I get the attached popup saying that the cert can't
be verified because it's unable to get the local issuer certificate.

In the past, when I (rightly) got this message, I'd click on "Always
accept", and I wouldn't get the warning any more unless I changed cert.

With the Let's Encrypt cert, "Always accept" doesn't seem to do the same
thing, as I have it pop up every time I start Sylpheed.

Optimally, I'd like to import Let's Encrypt's root cert into Sylpheed's DB
so I don't get these at all.  I found a website from 2007 that covered 
importing Gmail's certs into Sylpheed(*), but I'm hesitant to take advice
from a 13 year old site.

Does anyone have a clue how I can have Sylpheed accept this cert and not
bother me with popups?  Thanks in advance!

(*) http://www.melvilletheatre.com/articles/sylpheed-gmail/index.html

-- 
======================================================================
       Joe Hartley - UNIX/network Consultant - [email protected]
 Without deviation from the norm, "progress" is not possible. - FZappa