HubSpot will validate the email address used for any process that would create or update a contact record.  This includes using the Contacts API to create or update a contact directly, and also the processing of any form submissions or events.

These processes do not check the email address to make sure is is a valid email address (like an embedded form would), but the format of the address will be validated.  In addition to being valid according to RFC 2822, we place the following restrictions on email addresses:

• We don't allow the local parts to be quoted. (E.g. "Test Email"@hubspot.com is not valid)
• The domain part must end in a valid TLD, as listed at https://data.iana.org/TLD/tlds-alpha-by-domain.txt (note that this is after handling unicode TLDs, so "user@email.삼성" is a valid email address.
• When using the v3 Forms API, validation can be ignored by adding  the skipValidation parameter to the request body and setting it to true,