you could inform your users that email addresses are case sensitive much like passwords usually are or just normalize the local part along with the host and use that as the primary ID and have less issues in the future
servers that have case sensitive mailboxes are more likely to be used for throwaways or the user may control the server anyways so they could still respond to normalized local parts
as for verifying the email at registration you could check to see what the remote smtp server responds to when you issue the RCTP command to check if they consider the email valid
you could inform your users that email addresses are case sensitive much like passwords usually are or just normalize the local part along with the host and use that as the primary ID and have less issues in the future
We could normalise, and as far as I'm aware none of the largest e-mail services allow distinctions based on case so most of the time it would be OK. It would still be a security risk, though.
as for verifying the email at registration you could check to see what the remote smtp server responds to when you issue the RCTP command to check if they consider the email valid
You can, but many servers including some of the major services will just return a false positive for any mailbox to prevent that technique being used to collect addresses to spam, and even those that don't may consider such requests when not followed with a real message to be a black mark on the sender.
servers that have case sensitive mailboxes are more likely to be used for throwaways or the user may control the server anyways so they could still respond to normalized local parts
as for verifying the email at registration you could check to see what the remote smtp server responds to when you issue the RCTP command to check if they consider the email valid