Attachments and message size

The largest email Mailyoto accepts, the file types it refuses, and the limits for files sent through the API.

Updated 27 October 2026

These limits apply to email Mailyoto forwards and to email you send through it. They are the same on every plan.

Size#

Limit	Value
Largest email, over SMTP	`52,428,800` bytes (50 MB) once encoded, attachments included
Recipients per email, over SMTP	`100`
Files per email, through the API	`20`
Recipients per email, through the API	`50`, counting `to`, `cc` and `bcc` together
Largest email, through the API	`20 MB` once encoded, attachments included

The size counts the whole email as it travels: headers, text and every attachment. Email encodes attachments as base64, which makes each file about 37% larger. The files in one email can total about 36 MB.

Mailyoto announces the limit in its EHLO response, as SIZE 52428800. A sending server that reads it returns an oversized email to its sender without sending it.

The destination mailbox has its own size limit, often lower than Mailyoto’s. When it refuses a large email, the refusal shows in your logs and the sender gets a bounce. Send a link to a large file instead.

File types Mailyoto refuses#

Mailyoto checks the last extension of every attached file, including files inside an attached email. A trailing dot or space doesn’t hide the extension: invoice.exe. counts as .exe. When one file has a type below, Mailyoto refuses the whole email.

Kind	Extensions
Programs and app packages	`.apk` `.appx` `.appxbundle` `.com` `.dmg` `.exe` `.jar` `.jnlp` `.msix` `.msixbundle` `.pif` `.scr`
Installers	`.msi` `.msp` `.mst`
Scripts	`.bat` `.cmd` `.hta` `.js` `.jse` `.mjs` `.nsh` `.ps1` `.sct` `.vb` `.vbe` `.vbs` `.wsc` `.wsf` `.wsh`
Disk images	`.img` `.iso` `.vhd` `.vhdx`
Office add-ins and OneNote	`.one` `.onepkg` `.xll`
Windows system files	`.cpl` `.dll` `.lib` `.sys` `.vxd`
Shortcuts and settings files	`.ins` `.isp` `.library-ms` `.lnk` `.msc` `.reg` `.scf` `.searchconnector-ms` `.shb` `.url`
Cabinets, troubleshooters and help	`.cab` `.chm` `.diagcab` `.diagcfg` `.diagpkg`
Database programs	`.ade` `.adp` `.mde`

Each of these types runs code when opened, or carries files that do. Disk images, OneNote files and app packages are common ways attackers deliver malware.

The sending server gets 554 5.6.1 A blocked attachment type was rejected. This applies to email arriving for your aliases and to email you send over SMTP.

The Sending API refuses every type on this list, plus scripts such as .py and .pl. It responds 400 with the file’s name, for example We block .exe attachments. Remove report.exe.

To share one of these files, send a link to it instead.

Responses you may see#

Response	Meaning
`552 5.3.4 We take messages up to {limit} bytes, and this one declares {size}.`	The sending server announced a size above the limit before sending.
`552 5.3.4 Error: Too much mail data`	The email passed the limit while it was being sent.
`554 5.6.1 A blocked attachment type was rejected.`	One file has a refused type.
`Your account is limited to 20 attachments per email.`	An API request had more than 20 files.
`This email comes to 23.4 MB once encoded.`	An API request built an email over 20 MB. Base64 makes each file about a third larger.

Webhooks#

A webhook gets attachments in the JSON body, base64-encoded. Add include_attachments=false to the URL to get each file’s name, type and size without its content.