A preview is not a conforming message

Answer A preview is not a conforming message. RFC 2049 sets conformance criteria for MIME, the format that lets a message carry types, encodings, and parts. The point is that other software can tell what the message is, show the plain text, and deal with a type it does not know without crashing or running it. A seller's preview can pass that seller's eyes and still be a message a mailbox mangles. Approve the received copy.

The note that started this: RFC 2049, MIME Conformance Criteria and Examples, RFC 2049.

Solo ads are built in editors that hide the format. The editor shows a card. The card is not the bytes. If you are comparing best solo ad vendors, ask them to send the creative to a mailbox you control, through the same path the buyers will see. Then read it without their editor open.

Conformance is a low bar and a useful one. It does not mean the design is beautiful. It means a careful program can identify the parts and can refuse to treat an unknown part as a program to run. Mail that only renders inside one vendor's iframe has not been shown to clear even that bar. You do not need to memorize the criteria. You need the test that stands in for them.

What does MIME conformance ask of a message?

RFC 2049 sets criteria so mail software can recognize a MIME message and handle types it does not understand without crashing or executing them. A conforming message is one other software can read safely. It is not a screenshot.

Safely is the word buyers skip. An unrecognized part should be offered as a file or skipped, not launched. A creative that depends on a type only one client understands will look blank everywhere else, or worse, will ask the reader to open something they should not open. The practical reading of the document, for a desk that buys mail, is this. The offer has to exist as text a plain client can show. Anything else is decoration that might not travel.

Encodings are part of the same promise. A message can be written so the bytes survive a path that only carries a narrow character range, or it can assume every hop is generous. When the assumption is wrong, you get question marks where the offer was. That failure is a conformance and transport problem. It is not a mood. Look at the received source before you rewrite the headline.

• A test is received at a mailbox you open.
• The offer is readable without the vendor's preview.
• Unknown parts are not required for the pitch.
• The seller does not call a screenshot a send.

Is the seller's preview enough to approve a solo ad?

No. The preview is their tool, on their account. Ask for a copy received at a mailbox you open. If the offer is unreadable there, the preview was a private view.

Private views are how bad creatives get approved. The image loads because their editor inlines it. The button works because their preview rewrites the link. The plain-text part is empty because nobody on their team reads plain text. A stranger's phone will not extend those courtesies. One received copy, on a major web mailbox and on a phone, is a cheaper test than a drop you have to explain later.

Ask them to forward the raw message, not a picture of it. A picture cannot show a missing part. The raw message can. If they will not export it, they may not know what they are sending. That is a reason to pause the order, not a reason to trust the card on the call.

Does a conforming message reach the inbox?

No. Conformance is about whether the message is well formed enough for software to handle. Placement is a separate decision. A readable test is the floor. It is not a delivery promise.

A well-formed unwanted letter is still unwanted. Providers can file it, defer it, or refuse it for reasons that have nothing to do with MIME. Do not let a seller trade a clean preview for a placement claim. Pay for a list that fits, on a domain you can name, with a creative you have seen arrive. The document is the floor under the creative. Safe solo ads sellers can show you the floor. They do not get to call the floor the inbox.

RFC 2049 is a set of criteria for software and for the messages that software must tolerate. Use the spirit of it on the order. If you cannot read the received copy, you have not approved the send.