Every mail provider has its own filters, and a sender's reputation builds over time from recipients' complaints and reactions. Part of it can be checked technically, though, and it is usually the first thing to break delivery: authentication, the address mail leaves from, blacklists, encryption and the form of the message. This guide goes through them one by one. The delivery test checks them all on a real message: you get an address, send it a message and see what the receiving server sees.
What the large providers require
Gmail has published explicit requirements for whoever sends it mail (the "Email sender guidelines" page, read on 25 September 2026). For all senders:
- SPF or DKIM for the sending domain;
- valid forward and reverse (PTR) DNS records for the sending domain or IP address;
- a TLS connection for transmitting mail;
- messages formatted according to RFC 5322;
- a spam complaint rate, as reported in Postmaster Tools, below 0.3%.
Those who send more than 5,000 messages a day to Gmail must also have: SPF and DKIM, a DMARC policy (which may be p=none), the From domain aligned with the SPF or DKIM domain, and marketing and subscribed messages must support one-click unsubscribe, with a clearly visible unsubscribe link in the body. Senders who do not meet the requirements risk rate limiting, blocked messages or messages marked as spam.
SPF, DKIM and DMARC
- SPF (RFC 7208) says which servers may send for the domain of the envelope address (Return-Path). A server missing from SPF gets
softfailorfail. - DKIM (RFC 6376) signs the message with a key of the domain. The signature survives forwarding; SPF does not (RFC 7208, section 10.3).
- DMARC (RFC 7489) ties SPF and DKIM to the From address: the message passes only if one of them passes and is aligned with the From domain.
The usual mistake: an outside service (newsletters, billing, a CRM) that sends with your domain in From, but signs with its own domain and sends from its own SPF. SPF and DKIM pass, DMARC fails. The fix: DKIM with your domain in that service. Tools: the SPF generator, the DKIM generator, the DMARC generator; the DMARC step by step guide shows the gradual move to a strict policy.
Reverse DNS
The sending server's IP address must have a PTR record, and the name in it must lead back to the same address. RFC 1912 (section 2.1) asks that every address have a PTR matching the A record, since otherwise services may behave as for an unregistered name. Gmail requires it explicitly. The PTR is set by whoever holds the IP address (the hosting or internet provider), not in the domain's DNS panel. Check it with the SMTP server check.
Blacklists
Many servers ask, for every connection, public lists of addresses that sent spam (DNSBLs, RFC 5782). A listed address has its mail rejected or sent to spam wherever the list is used. Possible causes: a compromised account that sent spam, a web form abused to send mail, an IP address inherited from someone who spammed. Check the address with the blacklist check, fix the cause, then ask each list for removal through its own procedure.
TLS
STARTTLS (RFC 3207) encrypts the conversation between servers. Gmail requires a TLS connection for transmitting mail, so a server sending to it has to use STARTTLS. The delivery test shows whether the message arrived encrypted and with which TLS version.
The header
RFC 5322 (section 3.6) requires a Date field and the originator field (From) in every message, and section 3.6.4 says every message should also have a Message-ID. Gmail requires messages formatted according to RFC 5322, so a message from an application that forgets Date or From does not meet its requirements. From must appear once: messages with two From fields are typically rejected by servers that apply DMARC, since RFC 5322 forbids the form (RFC 7489, section 6.6.1).
Unsubscribing
The List-Unsubscribe field (RFC 2369) gives mail clients a standard way to unsubscribe. RFC 8058 adds one-click unsubscribe: besides an HTTPS URI in List-Unsubscribe, the message carries List-Unsubscribe-Post: List-Unsubscribe=One-Click, and the DKIM signature must cover both fields. The reason, as RFC 8058 itself puts it: when unsubscribing is too hard, the recipient reports the mail as junk, and senders whose mail is often reported that way tend to have poor deliverability.
What one message cannot show
The reputation of the domain and the IP address, the complaint rate, a sudden volume from a new address and the content (links to poorly reputed domains, images only, no text) matter too, but differ between providers and cannot be checked from a test message. For Gmail, Postmaster Tools shows reputation and complaint rate.
The steps, in short
- If you got an error message (a bounce), paste it here: its code, such as
550 5.7.26, usually says exactly which check failed. - Send a real message to the delivery test, from the system that has the problem.
- Fix what shows red: SPF, DKIM, DMARC, PTR, blacklists.
- Repeat the test until every check passes, then watch the DMARC reports for a few weeks.