Am 19.03.2020 um 14:29 schrieb A. Dumas:

>
> More or less impossible. E.g. apparently you didn't think that + is a
> valid character, which it is (in the part before the @). Also, domains
> (and usernames) can be UTF8. Best way is: try to deliver, check reply.

We are using a small CRM that checks if there exists a MX record in the
DNS for the domain part.

So, first check if domain is valid for e-mail, then try to deliver and
check response ...

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