Doesn't look like PHP problem.
Please update c-client to the latest available version and rebuild PHP.

Already done, I would not have posted here otherwise.

An abort in c-client still isn't something PHP can fix.
Please report this problem to c-client developers. 
See http://www.washington.edu/imap/

I have and this is their response.

Increasing the SENDBUFLEN to a sufficient size will make the "rfc822.c legacy routine buffer overflow" fatal error go away.  However, a better thing to do is to fix PHP to use c-client's new rfc822 header routines which do not require a fixed buffer (they flush the current buffer as
needed) rather than the legacy interface.

>However, a better thing to do is to fix PHP to use c-client's new
> rfc822 header routines which do not require a fixed buffer (they
>flush the current buffer as needed) rather than the legacy interface.

That's not going to happen in PHP4 and I honestly saying I doubt it'll happen ever because it's extremely difficult to add support for a new c-client functionality without actually _requiring_ some c-client version, which is not an option for such a widely used application as PHP.

So until c-client is missing a way to know its version and is breaking its own API between the releases, I guess we'll avoid using the new functionality and will stay with what we have now.

Though that doesn't mean we wouldn't review such a patch (of course if it does not add any new requirements).