You can use the options file to override the default values of various parameters that apply to the MTA as a whole. In particular, the option file is used to establish sizes of the various tables into which the configuration and alias files are read. You can also use the options file to limit the size of messages accepted by the MTA, specify the number of channels allowed in the MTA configuration, set the number of rewrite rules allowed, and so on.
In option.dat, lines starting with #, ! or ; are treated as comment lines, even if the preceding line has a trailing \ meaning that it is being continued. This means that long options that may contain these characters, in particular delivery options, care has to be taken.
For delivery options, where a natural layout would lead to continuation lines starting with a # or !, there is a safe and neat work around.
For more information about the syntax of the options file, see the Sun Java System Messaging Server 6.3 Administration Reference.