Dr Teeth wrote:

I have several accounts set up in one identity.

Outlook does not have identities. Do you mean that you're really using
Outlook Express? Click HelpAbout and post what it says (app name and
version).

Most of the time this works well.

They are A, B, and C; A being the default.

Sometimes, if I am using account C for a line of correspondence, a
reply will be sent from account A or B (but not always).

How can I keep Outlook from changing accounts like this? I sometimes
get caught out and only realise it has happened after a reply has been
sent.

Since Outlook replies using the same account as received the original
message, the usual cause of this is that the accounts reference the same
mailbox; i.e., that the mail addresses for the accounts are really aliases
of each other. Do your accounts all use different usernames/passwords to
connect to the mail server(s)?
--
Brian Tillman