[2038AD]

>But at least you have a brand new customer: [e-mail]. Since you do not lowercase the email and do not recognise him as [e-mail] which he used in his last order

You may not like it but this is fully standards-compliant behaviour. RFC 5321 states:

>The local-part of a mailbox MUST BE treated as case sensitive

I'm not saying this is a good behaviour and the RFC also discourages exploiting it further.

There are more ways an email address can be equivalent though :-). Since we know it's gmail in this case we know the email address is both case insensitive and dots don't matter. Comments are also allowed in email addresses (in both the local part and the domain part). Here's a couple examples from RFC 2822:

  pete(his account)@silly.test(his host)
  c@(Chris's host.)public.example

[0] https://tools.ietf.org/html/rfc5321

[1] https://support.google.com/mail/answer/7436150

[2] https://www.ietf.org/rfc/rfc2822.html#appendix-A.5