Last updated: July 2026

In short: Unlike many major cloud providers, Contabo does not document a general network-level block on port 25 across its help center. However, every server is subject to a strict outbound sending limit of approximately 25 emails per minute, and exceeding this threshold can trigger server throttling.

When hosting a virtual private server (VPS) or dedicated server with Contabo for outbound mail delivery, understanding the provider’s network rules is essential. While competing providers routinely block port 25 and require manual unblock requests, Contabo’s official documentation frames its restrictions around volumetric sending limits rather than a documented port ban.

Undocumented port status vs. strict sending limits

Within Contabo’s official help center, there is no documented mention of an outright default block on outbound port 25. However, this absence of documentation should not be mistaken for a guaranteed open port across all subnets.

Instead of documenting port blocks, Contabo strictly governs outbound email traffic through a per-minute rate limit. According to the provider’s official guidelines: The email sending limit is about 25 emails per minute, which should be plenty for running daily operations.

While 25 messages per minute is sufficient for standard administrative alerts or transactional notifications for small applications, it quickly becomes a bottleneck during marketing blasts or high-volume bursts.

Throttling and anti-spam enforcement

Contabo actively enforces its sending threshold to prevent bulk spamming from its infrastructure. Exceeding the rate limit or sending unsolicited mail violates terms of service and triggers immediate penalties: Spamming is against the Contabo Terms & Conditions and sending over the 25 emails per minute limit regularly may cause your server to be throttled.

IP reputation reality check

Even if your mail volume stays well below 25 emails per minute and port 25 connects cleanly, self-hosting direct email delivery on Contabo faces significant deliverability hurdles. Because Contabo offers budget-tier VPS hosting, their IP blocks are frequently targeted by bad actors. Consequently, large Contabo subnets regularly appear on strict blocklists (DNSBLs) such as UCEPROTECT (Level 2 or 3) and Spamhaus.

If you attempt direct SMTP delivery from a Contabo IP to major receivers like Gmail, Yahoo, or Microsoft 365, messages often end up in spam folders or face outright rejections due to poor subnet reputation, regardless of perfect SPF and DKIM alignment.

Smarthost relaying via port 587

To bypass both the 25 emails/minute rate limit and the negative reputation of shared VPS IP blocks, you should route your outbound mail through an authenticated external smarthost relay. Relaying over port 587 isolates your delivery from IP blocklists and volumetric throttling.

A standard configuration in /etc/postfix/main.cf for relaying through an external smarthost looks like this:

relayhost = [smtp.provider.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt

Or send reliably via Dispatch. If you want to avoid rate limits, blocklist challenges, and smarthost maintenance entirely, you can route your outbound mail securely and reliably via Dispatch.

Verifying your configuration

Once your sending architecture is operational, audit your email authentication headers (SPF, DKIM, DMARC) and check for domain blocklisting using the free MXAudit scanner.

Further reading