Since 2.1.17 fixes a lot of bugs of 2.1.7, I would propose you to open a bug report on the dovecot package in bugs.debian.org and list some of the bugs which have been fixed in the meantime. The easiest way to do so is to use the "reportbug" tool in Debian.
At least then there is some hope to get a known-to-be-stable release into some of the next Debian (and Ubuntu) releases. Regards Daniel
