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Information About Sending E-mails to 1&1 Mailboxes


In this article, we have compiled information for you about sending e-mails to 1&1 mail servers.

Tips for users

I have set up my e-mail program but cannot deliver any emails to 1&1.
Please check your e-mail program settings using your provider's recommended settings. Also make a note of the error messages and try a different approach to delivering the e-mail.

What is the maximum size for attachments that I send to 1&1 users?
The maximum size of an e-mail is 100 MB. However, please note that in the case of encrypted transmission, the file size must not exceed 72 MB as the remainder is required for encryption. We do not recommend sending e-mails with attachments of this size. This can cause extremely long download times and/or cause various error messages.

Why are my e-mails not being delivered?
Whenever an e-mail cannot be delivered, 1&1 always sends an error message to the original sender. Please refer to the list of error messages to discover the reason why your e-mail could not be sent.

My e-mails are not being delivered. Why am I not seeing an error message?
If an e-mail cannot be delivered, 1&1 always sends an error message to the original sender. Please check whether the error message was identified as spam or moved to another folder due to a filter rule. Please also check whether the e-mail system that you use actually forwards error messages to users.

Tips for Administrators

Why is their a delay in delivering my e-mails?
Since e-mail is an asynchronous medium, we and all other providers cannot guarantee prompt delivery. However, we do have a large team of specialists who work on ensuring trouble-free e-mail correspondence.

Why are my e-mails being identified as spam?
The 1&1 spam filter systems comprise a number of different IP-based and content-based filter modules. As a result, an e-mail may be incorrectly identified as spam. Please send us the extended header for any e-mail of this kind. Please note that e-mails that are organized in the “Unknown” folder have been rated with a low spam score.

How can I ensure that my e-mails are not identified as spam?
For more information, refer to our recommendations for sending e-mails.

My systems have just recovered from a malfunction. Why are my e-mails still being rejected?
When our systems detect that spam is being sent from specific servers, IP-based blocking of the compromised systems comes into effect. Please use this contact form so that we can manually unblock your systems.

How can I get on the 1&1 whitelist?
1&1 does not offer this service. Please refer to our recommendations for sending e-mails. If you observe these recommendations, whitelisting is unnecessary.

Does 1&1 use graylisting?
In the event of unmistakable characteristics that indicate a spamming server, we delay any e-mails sent from a specific IP address, or reject them completely. Please refer to our recommendations for sending e-mails in this regard. If you observe these references, we can rule out any delayed acceptance of your e-mails.

Why are connections between my server and the 1&1 systems being refused?
Please check whether your e-mail server IP address is listed on an established RBL list. You can check this here. Please contact the list operator directly if this is the case.

Additionally, check your server logfiles as of the first occurrence of any error.

Check whether you comply with the standards defined in the RFCs. This includes providing a syntactically correct sending address, the use of HELOs that return the IP address in a reverse lookup, and setting up a reverse DNS entry (FQDN) for your e-mail server IP address.

Your systems may have been misused to send e-mails by unauthorized persons. Secure your systems against unauthorized use.

If you send newsletters or bulk e-mails, check your delivery quota and remove any e-mail addresses that no longer exist from your database. You must have the explicit permission of all recipients to whom you send e-mails.

Please contact us to discuss the situation and/or unblock your server. Please send us relevant logfile entries and a description of the error that occurred, including the error messages. Please use our contact form to do so.