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What kind of problem have you encountered?

Does your domain provided in MAIL FROM address use SPF policy?

Frequently Asked Questions

554 - Message refused by the scanner

Click on “E-mail” to create a new email (or attach to any reply). Select the email you want to forward in the folder, and then drag it to the body area of the email.

General emails: Send us the refused email as an attachment to samples@virusfree.cz. Or send the email to the above address, as though you were sending it to the original recipient.

421 - Service temporarily not available

SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail.
PTR (Pointer records) are used to map a network interface (IP) to a host name. These are primarily used for reverse DNS.

This announcement shows that you have likely reached the maximum limit for parallel SMTP connections. You should also check the PTR settings of your IP address and its matching A records. You can check your PTR setting here.

Connection timed out

You have exceeded the new connections limit. Send further emails via a single TCP/SMTP connection.

DNS/PTR record point to A record that does not point to your IP (see RFC1912)

Please read RFC 1912 section 2.1 and RFC 5321 section about SMTP communication.

IP listed on BIP.virusfree.cz, BAD.virusfree.cz

Your sending IP address is listed because you sent only one email calssified as spam. For delisting, send an email from the listed IP to samples@virusfree.cz. The delisting process is automatic after sending one single clean email. Your IP will be removed the following day.

Bulk Senders Guidelines


  • Mailer’s domain must have valid SPF
  • Mailer’s domain must have reverse DNS records by IP addresses domains used for sending the email (Please read RFC 1912 section 2.1)
  • Collective emails must use dedicated IP addresses
  • The “From” field in all email headers must remain the same

Newsletter unsubscribes

  • Users must be provided with the opportunity to unsubscribe from a newsletter with a single click
  • The “unsubscribe” option must be contained in the main content of the message
  • The link for unsubscribing should direct the user to a webpage, where they can unsubscribe. To unsubscribe, the user must not have to submit any personal data. All text relating to the ‘unsubscribe’ function must be viewable by users utilising plaintext settings
  • The “unsubscribe” function can also be set up so that users can unsubscribe by replying via email to the sender

Email format

  • Messages must comply with RFC 5322 standards
  • HTML messages must comply with W3C standards
  • Messages must be marked with the email header 'Precedence: bulk'
  • Sent messages attempts that result in the original sender or webpage address being hidden won’t be delivered
  • The message subject should not be misleading and must accurately represent the message content

Sending Collective Messages Rules

  • For efficient processing, please send collective emails between 10 PM and 5 AM UTC, if possible.
  • Maximum message size is 120kB / message
  • There is a maximum limit of 15 parallel connections from all mailing IP addresses
  • IP address connections for sending collective emails, where will be in one TCP connection just one email, will not be accepted
  • During a single SMTP connection, it is possible to send a maximum of 1000 messages
  • To ensure that all messages are received, there is an established limit of 300 messages per minute

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Excello s.r.o

CRN: 27444899

VAT number: CZ27444899

Prague, Czechia

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