Lets be totally correct, avast didn't block it, but alerted you to an infection/suspicion on an email; you the user must input what action avast takes. What action did you select ?
If you deleted it, it is history gone.
If you sent it to the chest, that too I think is unrecoverable as trying to place the infected email back in to the inbox may well corrupt the email database file, which could result in the loss of all other emails in that database.
If you chose Ignore it should have gone to your inbox.
What is your email program ?
Was this from a webmail source, like hotmail, view through your browser ?
It may be easiest to contact that person and ask a) if they sent something, they might not have sent it and their system could be infected and b) if they did send it ask them to resend it.