a...bcd@domain.com is malformed.
The email address is an RFC 2822 addr-spec which is defined as
addr-spec = local-part "@" domain ; global address
local-part = word *("." word) ; uninterpreted
; case-preserved
A local part is any number of <word>s separated by ".". A word is either a string of non-specials ("." is a special) or a double quoted string possibly containing specials other than dquote, so you can have dots between words but not adjacent and not at the beginning or end.
The + character is allowed though.