I've had a complaint from a customer that they can't reply to our user generated email addresses - there are various reasons for the format below, but the long and short of it is that the Yahoo! Mail web interface doesn't like it and just spits out an error:

We cannot find a match for this email address:


Please use an email address that looks like [email protected] or a nickname or a distribution list name from your Yahoo! contacts.

Does anyone know why this isn't passing the Yahoo! Mail filter?

1 Answer 1


It appears the local part / username of the email address must be 64 characters or less for it to be accepted by Yahoo! Mail.

This restriction is not seemingly enforced by all providers, but it is in the spec here:


The format of email addresses is local-part@domain where the local-part may be up to 64 characters long and the domain name may have a maximum of 253 characters - but the maximum 256 characters length of a forward or reverse path restricts the entire email address to be no more than 254 characters

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