There are no dates in the SMTP protocol, but you are probably referring to email header fields as described by RFC 2822 Internet Message Format. From http://www.ietf.org/rfc/rfc2822.txt we have section 3.3 Date and Time specification, with the following selected fields:
date-time = [ day-of-week "," ] date FWS time [CFWS]
day-of-week = ([FWS] day-name) / obs-day-of-week
day-name = "Mon" / "Tue" / "Wed" / "Thu" /
"Fri" / "Sat" / "Sun"
date = day month year
year = 4*DIGIT / obs-year
month = (FWS month-name FWS) / obs-month
month-name = "Jan" / "Feb" / "Mar" / "Apr" /
"May" / "Jun" / "Jul" / "Aug" /
"Sep" / "Oct" / "Nov" / "Dec"
day = ([FWS] 1*2DIGIT) / obs-day
...
which states that the day part of the date should be between 1-2 digits, and this does not specifically prohibit using a leading zero. Nothing seems to be said about the optional leading zero, so proper interpretation should always allow it while parsing.
The obs- alternative variants describe old obsolete formats, e.g. 2-digit years and comments or folding white-space between any part.