If it's a single-domain hosted email provider, a well-designed system would accept either the local part or the full email address, but using the full email add

If it's a single-domain hosted email provider, a well-designed system would accept either the local part or the full email address, but using the full email address would avoid problems in the future in case the service expands to a second domain. The local part of an email address is not an email address. If it asks for an email address during login, assume it means an email address.