I want to replace postfix with exim4 on my Ubuntu test server to reflect the setup I have on my production server, but apt-get and aptitude (quite understandably) insist on removing several packages that depend upon having a mail stack. However, in this case I am prepared to override apt-get's undoubted good sense as exim should fulfil all the requirements of the dependent packages, providing mail and sendmail functionality for my applications. I don't want to remove the dependent packages as there is months of effort invested and, although backed up, will be a pain to reconstruct properly.