With the upgrade of Raspbian from stretch to buster I lost SSL support
in sylpheed with version "3.7.0" .

So in order to regain sylpheed SSL on buster I installed the following
Jessie items as follows:-

sudo apt-get remove sylpheed

sudo dpkg -i libssl1.0.0_1.0.1t-1+deb8u12_armhf.deb
sudo dpkg -i libonig2_5.9.5-3.2+deb8u3_armhf.deb
sudo dpkg -i sylpheed_3.5.0_beta1_r3426-1_armhf.deb
sudo dpkg -i sylpheed-plugins_3.5.0_beta1_r3426-1_armhf.deb

sudo apt-mark hold sylpheed

When you feel ready to update sylpheed again you can do the following:-

sudo apt-mark unhold sylpheed