Sounds like the problem is internal to the milter-greylist process, not some outside effect, but still possibly some other library it's using.
Which version of Debian is it?
The openssh part reminds me: DKIM depends on crypto (libssl I believe), so maybe an openssl update messed with it. Or maybe it was ssh; looks like there's a dependency chain of milter-greylist -> libcurl3 -> libssh2. Though I'm not clear on why milter-greylist would need libcurl.
If it were me I'd be seriously questioning why my greylist daemon is checking DKIM. Can you turn it off with "dkim none" in greylist.conf? If not, at this point I think you'd be justified with either the rebuild (causing problems with updates) or just switching greylist daemons.
Re: Thoughts & Ideas
Which version of Debian is it?
The openssh part reminds me: DKIM depends on crypto (libssl I believe), so maybe an openssl update messed with it. Or maybe it was ssh; looks like there's a dependency chain of milter-greylist -> libcurl3 -> libssh2. Though I'm not clear on why milter-greylist would need libcurl.
If it were me I'd be seriously questioning why my greylist daemon is checking DKIM. Can you turn it off with "dkim none" in greylist.conf? If not, at this point I think you'd be justified with either the rebuild (causing problems with updates) or just switching greylist daemons.