553 5.1.3 "Not a Valid RFC 5321 Address": Fix the Data, Not the Server

Sources, read 3 November 2026: Google's Gmail SMTP errors and codes, RFC 3463, RFC 5321 section 4.5.3.1 (size limits) and RFC 6531 (SMTPUTF8).

What the code says

RFC 3463 defines X.1.3 as "Bad destination mailbox address syntax": "The destination address was syntactically invalid. This can apply to any field in the address." The code page for 553 5.1.3 has the variants.

What breaks addresses in a list

The first three come from merges and scrapes; the last from a CSV opened and saved in the wrong encoding.

The limits in the RFCs

RFC 5321 sets maximum lengths: 64 octets for the local part (before the @) and 255 for the domain. Lists rarely hit these honestly; when they do, two fields have usually been glued together. Which characters are allowed is covered in valid email address characters.

Addresses with non-ASCII characters before the @ are a separate case. They're legal under RFC 6531, but only between servers that both support the SMTPUTF8 extension. See internationalized email addresses.

What to do

1. Copy the address from the bounce. Gmail inserts the exact string it rejected, including the invisible problems.
2. Find the row in your source file and look at the raw cell, not the display in a spreadsheet.
3. Fix it using the table above, then search the whole list for the same pattern. One mailto: usually means a column full of them.
4. Suppress the malformed string in your sending tool so it isn't retried.
5. Run a syntax pass on the whole list before the next send (below).

Find every broken row first

1. Bulk email syntax checker flags malformed addresses with the reason, free, in your browser.
2. Split multiple emails in a cell turns a; b cells into separate rows.
3. CSV encoding fixer repairs ñ-style characters; fix garbled characters in a CSV explains the cause.

Regex alone isn't the answer for the edge cases; email validation regex explains where a pattern stops being useful.

Limits

A syntactically valid address can still be dead. Syntax checks only remove rows that can't possibly deliver; they don't tell you whether the mailbox exists.

Where SimpleVerifier fits

Verification runs the syntax check first and then checks the domain and the mailbox, so malformed rows come back invalid with the reason and the rest get a real answer. Upload the list at /verify; the first 100 checks are free with a Google sign-in. Pro is $39 a month with the first 100,000 checks a month at full speed, then slower, never cut off. See pricing.