I have a 3 character .com as my primary email... it gets rejected more often than I'd like... including at my bank :) I've got a longer more normal domain that I alias, but it annoys me none the less.

Have you got this lot sorted out:

  MX->A->PTR->A->MX
  SPF
  DKIM
  DMARC
  mta-sts - DNS and webpage