feat(mail): handle multiple mail identities

If no custom identity exists, initialize the mail identities of the user
with all the user's email addresses (extracted from MailFieldNames).

For this code path to be executed, the user must not have any of the
following defaults keys:

  SOGoMailIdentities
  SOGoMailCustomFullName
  SOGoMailCustomEmail
  SOGoMailReplyTo
  SOGoMailSignature

Fixes #768, fixes #4602

SoObjects/SOGo/SOGoUser.m

@@ -786,20 +786,28 @@ - (void) _appendSystemMailAccountWithDelegatedIdentities: (BOOL) appendDeletegat
 
   if (![identities count])
     {
-      // Create a default identity
-      identity = [NSMutableDictionary dictionaryWithObjectsAndKeys:
-                                        fullName, @"fullName",
-                                      [self systemEmail], @"email", nil];
-      if (appendDeletegatedIdentities)
+      // Create an identity for each email address
+      max = [mails count];
+      for (count = 0; count < max; count++)
         {
-          [identity setObject: [NSNumber numberWithBool: YES] forKey: @"isDefault"];
-          hasDefaultIdentity = YES;
-        }
-      else
-        {
-          [identity setObject: [NSNumber numberWithBool: YES] forKey: @"isReadOnly"];
+          identity = [NSMutableDictionary dictionaryWithObjectsAndKeys:
+                                            fullName, @"fullName",
+                                                  [mails objectAtIndex: count], @"email", nil];
+          if (appendDeletegatedIdentities)
+            {
+              if (count == 0)
+                {
+                  // First identity uses the system email -- mark it as the default
+                  [identity setObject: [NSNumber numberWithBool: YES] forKey: @"isDefault"];
+                  hasDefaultIdentity = YES;
+                }
+            }
+          else
+            {
+              [identity setObject: [NSNumber numberWithBool: YES] forKey: @"isReadOnly"];
+            }
+          [identities addObject: identity];
         }
-      [identities addObject: identity];
     }
 
   /* identities from delegators */