Enigmail Help
Using the Enigmail Rules Editor: Edit OpenPGP Rule
In the Rules Editor, you can specify defaults per recipient for enabling 
encryption, signing and PGP/MIME, and to define what OpenPGP key(s) to use. In this 
dialog, you can specify the rules for a single recipient, and for a group 
of recipients with very similar attributes.
  - Set OpenPGP Rules for: Contains the email
    addresses of the recipients (without names, i.e. just an address like
    somebody@email.domain). You can specify several email addresses, separated by spaces.
    The address specified here can consist of
    only the domain section so that mail to any address at that domain will
    be matched, e.g. @email.domain will allow matching to body@email.domain,
    somebody@email.domain, anybody@email.domain, etc.
  
- Apply rule if recipient ...: 
    This modifies the matching of the email addresses.
    If multiple addresses are entered, the setting will apply to all.
    The examples below are based on body@email.domain
    entered in the OpenPGP Rules field above.
    
      - Is exactly: with this setting, the rule will only trigger on
       emails to body@email.domain (exact, case insensitive matching).
      
- Contains: with this setting, any email address containing the string
        is matched,
        e.g. anybody@email.domain or body@email.domain.net
      
- Begins with: with this setting, any email address starting with
        the string is matched, e.g. body@email.domain.net,
        body@email.domain-name.com.
      
- Ends with: with this setting, any email address ending with
        the string is matched, e.g. anybody@email.domain,
        somebody@email.domain.
      
 
- Continue with the next rule for the matching address
 Enabling this function will allow you to define a rule but not have 
	  to specify a KeyID in the Use the following OpenPGP keys: 
	  field, so that the email address is used to check for a key at the 
	  time of sending. Also, further rules for the same address(es) will
	  be processed as well.
- Do not check further rules for the matching address
 Enabling this function stops processing any other rules for the 
	  matching address(es) if this rule is matched; i.e. rule processing 
	  continues with the next recipient.
- Use the following OpenPGP keys:
 Use the Select Key(s).. button to select the recipient 
	  keys to be used for encryption. As in the action above, no further 
	  rules for the matching address(es) are processed.
- Default for Signing: enable or disable
    message signing. This either uses or overrides what you have specified
    in the message composition window. The values are:
  
    - Never: disable signing, even if it was enabled in the message
       composition window (overrules the other values)
 
- Yes, if selected from in Message Composition: leave signing 
      as specified in the message composition window
    
- Always: enable signing, even if it was not enabled in the message
      composition window
    
These signing settings are applied for
all rules that match. If one of the rules disables signing, the message
will not be signed, regardless of other rules that specify Always.
  - Encryption: enable or
    disable message encryption. The allowed settings and their meaning are
    the same as for message signing.
- PGP/MIME: enable or
    disable the use of the PGP/MIME (RFC 3156) message encoding. 
    If PGP/MIME is disabled, the
    messages are encoded using "inline PGP". The allowed values and their
    meaning are the same as for message signing.
  
The rules are processed in the order displayed in the list in the 
OpenPGP Rules Editor. Whenever a rule 
matches a recipient and contains a OpenPGP Key ID, in addition to using the 
specified Key ID, the recipient is not considered anymore when processing 
further rules.
Further help is available on the
Enigmail Per-Recipient Settings page