Seznam.cz and Email.cz Bulk Sender Rules: Authentication, Sending Limits and Contact Database Hygiene

Seznam, the Czech mail and search company, publishes a set of help pages for bulk senders, and the short version is: authenticate with SPF and DKIM, pace your sending per IP, send only to people who agreed, and delete contacts that don't exist. This post summarizes those pages in our words, as read on 9 November 2026 on Seznam's sender help. The pages are in Czech; where we describe them we are paraphrasing, so use the originals for anything you will rely on.

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Where Seznam's rules live

Seznam's mail help has a section for bulk senders ("Informace pro odesílatele zpráv" or, on the page URL, "pro odesílatele hromadných zpráv"). It has pages on limits, mandatory technical requirements, recommended domain settings, working with a contact database, the Feedback Loop, and further tips. Three of them matter for list owners: authentication, limits and the contact database.

Authentication

The page on technical requirements covers three records:

• SPF defines which servers may send mail for your domain. List every server that sends as your domain.
• DKIM shows that a message wasn't changed in transit and that the sender is authorized to use the domain in the signature.
• DMARC builds on both, helps stop misuse of your domain, and gives you reports about who sends on your behalf.

Seznam says valid SPF and DKIM records are needed for reliable delivery, and that when they are missing or don't match, messages can land in Spam or be discarded outright. It strongly recommends DMARC and notes that providers worldwide are moving towards requiring it. The page we read says nothing about PTR records, TLS or numeric thresholds, so we don't claim anything on those. Our posts on DKIM for every sending service and reverse DNS and PTR records cover the general setup, and the email domain health checker shows what your domain publishes today.

Sending limits

Seznam's page on mailing size limits explains that the limits protect its infrastructure and that following them is necessary for a bulk mailing to be delivered. The figures:

Limit	What the page says
Messages per SMTP connection	At most 100 (given as an example figure)
Connections per IP	No more than 1,500 in any five-minute interval
Messages per IP	No more than 100,000 in the same five-minute interval, with a total of 1 GB of data
Spreading a send	Distribute it over tens of minutes to several hours, depending on size

The limits apply per sending IP, and the five-minute window is measured for each IP separately. If you send through an email platform, the platform controls the connection settings, so ask support whether it throttles for Seznam. If you run your own server, set a per-domain rate for seznam.cz and email.cz recipients.

The FAQ adds that a message may fail because of the recipient address, limits on message count or size, too many failed connection attempts, or Seznam's anti-spam. It says a bounce from Mailer Daemon states the specific reason, so read it.

The contact database page

This is the page most relevant to a list owner. It asks senders to:

• Send only to contacts who gave valid consent.
• Use double opt-in for new contacts, which keeps invalid addresses out of the database and helps avoid spam traps.
• Show a visible unsubscribe link, for example in the footer, in line with Czech Act No. 480/2004 Sb., and it recommends one-click unsubscribe.
• Delete non-existent and inactive contacts, and recipients who marked your messages as spam.
• Use the Seznam Feedback Loop, free to register at fbl.seznam.cz, to learn who marked your mail as spam.
• Not use purchased contact databases.

The page also says sender reputation depends largely on how recipients reacted to your earlier messages: whether they read, deleted unread or reported them. It gives no numbers for bounce handling, complaint limits, how long a contact must be silent before counting as inactive, or what Seznam does to senders who ignore the advice. We don't invent them. A target to aim just below doesn't exist on these pages, so the safe approach is to send as little as you can to addresses that don't work.

Where verification fits

Authentication and rate limits are settings you configure once. The contact database changes every month. "Delete non-existent contacts" is a verification job:

• Valid: the mailbox accepts mail. Send.
• Invalid: the mailbox doesn't exist. Remove it from the list, permanently.
• Risky or unknown: the answer wasn't clear. Send these in a small, slower group after the valid ones went out without trouble.

Double opt-in prevents most typos at the door, but it can't help with the list you already have. For old lists, our post on how often to verify an email list gives a schedule, and cold email suppression lists shows a simple way to make sure removed addresses never return. How mailbox providers answer verification checks in 2026 explains why some answers come back risky and not definite.

"Inactive" is a different thing: someone whose mailbox works but who hasn't opened anything for a long time. A verifier can't see that. Use your own engagement data for it; our sunset policy generator helps you write the rule.

A routine for a Czech or Slovak list

1. Pull out the Seznam rows. Filter your list for the Seznam domains you see, such as seznam.cz and email.cz.
2. Verify the file at /verify, or check single addresses with the Seznam email checker.
3. Remove invalid rows and keep a record of them.
4. Send the valid rows in batches, spread over time and within the per-IP limits above.
5. Sign up for the Feedback Loop and remove anyone who complains.
6. Read every bounce from Mailer Daemon in full. If the meaning isn't clear, paste it into the bounce message analyzer.

Quick answers

• Is a 4xx a bounce? No. A 4xx is a temporary failure, and your server should retry on its own.
• Do I need DMARC? Seznam says it is strongly recommended. SPF and DKIM are the ones it describes as needed.
• Is there a fixed bounce-rate limit? Not on the pages we read.

Try it on your own list

Upload a sample at /verify. The first 100 checks are free with a Google sign-in, no card.

Pro, $39 a month: Unlimited email verification.* Never metered, never cut off. See pricing.

*Fair use: the first 100,000 checks a month at full speed, then slower, never cut off.