In your compose email view, at the top youll see 'Send Using: [Default]'
You can change this between the accounts registered to your device and as long as you 'reply' to each email it will continue to use the account that you specified with the first message.
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"All of the things I enjoy are immoral, illegal, or fattening" - Oscar Wilde
"All of the things I enjoy have a Wife Approval Factor (WAF) of 0" - Me