GPGME since version 1.7.0 provides TOFU support (time of first usage gpg trust policy).  Proposed changes add support of showing TOFU information in the Sylpheed-3.7 if it is available. 

First patch (sylpheed-gpgme-version-update.patch) changes gpgme version requirement to 1.7.0 and updates documentation accordingly.  Please note that it must be updated anyway, since Sylpheed's gpgme code uses key->fpr (the char *fpr field of the gpgme_key_t stucture), which is available only from gpgme-1.7.0 according to gpgme info documentation. 

GnuPG 2.1.x is required for TOFU as well, though if it is not available, gpgme will disable tofu support, so there is no need to force users to update GnuPG, some people are still preferring gpg-1.x series, so there is no need to force them to update. 

Second patch sylpheed-tofu.patch adds actual TOFU support by allowing users to see TOFU information after GnuPG signature status information.  It also adds more detailed explanation to the GnuPG status (e.g. shows signature trust level like marginal, full, ultimate trust). 

Works fine for me with Sylpheed-3.7, I'll be glad to see this new feature in the next Sylpheed relase.