Maximum Recipients
Each email can have a maximum of 50 recipients combined across the to, cc, and bcc fields. This limit applies to both the REST API and SMTP relay.
Field Description toPrimary recipients ccCarbon copy recipients bccBlind carbon copy recipients Total Maximum 50 combined
Emails exceeding 50 recipients (combined) will be rejected. For larger recipient lists, use the batch sending endpoint which supports up to 500 recipients per request.
This limit aligns with transactional email best practices. Transactional emails (receipts, notifications, password resets) are typically sent to a small number of recipients. If you need to send to larger lists, consider:
Using the batch API for sending the same email to multiple recipients
Splitting large recipient lists into multiple API calls
Using broadcast routes for marketing or newsletter-style emails
Email Size Limit
The maximum size for an email message (including all attachments) is 25MB . This limit includes:
The email body (HTML and plain text)
All attachments (base64 encoded)
Headers and metadata
Emails exceeding 25MB will be rejected with an error. Consider using external file hosting services for large files and including download links in your email instead.
Blocked Attachments
For security reasons, certain file types are blocked from being sent as attachments. These restrictions help protect recipients from potentially malicious files.
Blocked File Types
The following table lists all blocked file extensions and their associated MIME types, organized by category:
Executable Files & Applications
Extension MIME Type(s) Description .exeapplication/x-msdownload, application/x-executable, application/octet-stream-exeWindows executable .comapplication/x-msdownload, application/x-msdos-programDOS command file .appapplication/x-ms-applicationApplication file .msiapplication/x-msi, application/x-ms-installerWindows installer package .mspapplication/x-msiWindows installer patch .mstapplication/x-msiWindows installer transform .gadgetapplication/gadget, application/x-gadgetWindows gadget .dllapplication/x-msdownloadDynamic link library .jarapplication/java-archiveJava archive
Script Files
Extension MIME Type(s) Description .batapplication/x-bat, application/x-cmdBatch file .cmdapplication/x-cmd, application/x-batCommand script .ps1application/x-powershell, text/x-powershellPowerShell script .ps1xmlapplication/x-powershellPowerShell XML .ps2application/x-powershell, text/x-powershellPowerShell v2 script .ps2xmlapplication/x-powershellPowerShell v2 XML .psc1application/x-powershellPowerShell console file .psc2application/x-powershellPowerShell v2 console file .vbapplication/x-vb, text/vbscriptVisual Basic file .vbsapplication/x-vbs, text/vbscriptVBScript file .vbeapplication/x-vbe, text/vbscriptEncoded VBScript .jsapplication/javascript, application/x-javascript, text/javascript, text/x-scriptJavaScript file .jseapplication/x-javascript, text/jscriptEncoded JavaScript .wstext/x-scriptWindows script .wsftext/x-scriptWindows script file .wsctext/x-scriptWindows script component .wshtext/x-scriptWindows script host .shapplication/x-shellscript, application/x-shShell script .cshapplication/x-cshC shell script .kshapplication/x-shellscriptKorn shell script .aspapplication/x-asp, application/asp, text/asp, text/x-aspActive Server Page
System & Configuration Files
Extension MIME Type(s) Description .sysapplication/x-sys, application/octet-stream-sysSystem file .cplapplication/x-msdownloadControl panel item .scrapplication/x-msdownloadScreen saver .regapplication/x-regRegistry file .inftext/plainSetup information file .crtapplication/x-crtCertificate file .cerapplication/x-crtSecurity certificate .derapplication/x-crtDER certificate
Microsoft Office & Access
Extension MIME Type(s) Description .mdbapplication/x-msaccess, application/vnd.ms-accessAccess database .mdeapplication/x-msaccessAccess compiled database .mdaapplication/x-msaccessAccess add-in .mdtapplication/x-msaccessAccess data file .mdwapplication/x-msaccessAccess workgroup .mdzapplication/x-msaccessAccess wizard .accdbapplication/x-msaccessAccess 2007+ database .adeapplication/x-msaccessAccess project extension .adpapplication/x-msaccessAccess project .madapplication/x-msaccessAccess module .mafapplication/x-msaccessAccess file .magapplication/x-msaccessAccess diagram .mamapplication/x-msaccessAccess macro .maqapplication/x-msaccessAccess query .marapplication/x-msaccessAccess report .masapplication/x-msaccessAccess stored procedure .matapplication/x-msaccessAccess table .mauapplication/x-msaccessAccess user .mavapplication/x-msaccessAccess view .mawapplication/x-msaccessAccess web .pubapplication/x-mspublisherPublisher document .vsmacrosapplication/vnd.ms-vsmacrosVisual Studio macros
Web & Browser Files
Extension MIME Type(s) Description .htaapplication/htaHTML application .htm(when containing scripts) HTML file .html(when containing scripts) HTML file .urltext/x-urlInternet shortcut .mhtmlVarious Web archive
Help & Documentation
Extension MIME Type(s) Description .chmapplication/vnd.ms-htmlhelp, application/x-chmCompiled HTML help .hlpapplication/winhlpWindows help file .itsRelated to help files Internet document set
Shell & Shortcuts
Extension MIME Type(s) Description .lnkapplication/x-ms-shortcutWindows shortcut .pifapplication/x-pifProgram information file .shbShell file Shell scrap object .shsShell file Shell scrap object .scfapplication/x-explorer-commandWindows Explorer command .scttext/scriptletWindows script component .xnkExchange shortcut Exchange shortcut
Installer & Package Files
Extension MIME Type(s) Description .insapplication/x-internet-signupInternet settings .ispapplication/x-internet-signupInternet service provider .libapplication/x-archiveLibrary file .mscapplication/x-mscManagement console .opsOffice profile Office profile settings .pcdPhoto CD Photo CD image .prgProgram file Program file .fxpFoxPro FoxPro compiled
Visual Studio & Development
Extension MIME Type(s) Description .vssapplication/vnd.visioVisio stencil .vstapplication/vnd.visioVisio template .vswapplication/vnd.visioVisio workspace .vxdVirtual device Virtual device driver
Media Files
Extension MIME Type(s) Description .wmzapplication/x-ms-wmzWindows Media skin .emfapplication/x-msmetafileEnhanced metafile
FoxPro Files
Extension MIME Type(s) Description .basBASIC source BASIC source code .prfSettings file Profile settings .plgPlugin file Plugin file .mshScript shell Microsoft Shell .msh1Script shell Microsoft Shell v1 .msh2Script shell Microsoft Shell v2 .mshxmlScript shell XML Microsoft Shell XML .msh1xmlScript shell v1 XML Microsoft Shell v1 XML .msh2xmlScript shell v2 XML Microsoft Shell v2 XML
Temporary Files
Extension MIME Type(s) Description .tmpVarious Temporary file
Miscellaneous
Extension MIME Type(s) Description .applicationapplication/x-ms-applicationClickOnce deployment .obdOffice binder Office binder document .mvbMultimedia viewer Multimedia viewer file .vpsVirtual private server VPS file
Additional MIME Type Restrictions
The following MIME types are also blocked:
application/octet-stream (for executable files)application/x-msdownloadapplication/x-ms-installerapplication/x-msdos-programapplication/x-msscriptapplication/x-ms-shortcutapplication/x-ms-wmzapplication/x-msmetafileapplication/x-msbinderapplication/x-msmediaviewtext/scriptlet
Why These Restrictions?
These file types are blocked because they can:
Execute code automatically when opened
Exploit vulnerabilities in email clients or operating systems
Bypass security measures through social engineering
Contain macros or scripts that could harm the recipient's system
Be used to spread malware or ransomware
Most business documents (PDF, DOCX, XLSX), images (JPG, PNG, GIF), and common file formats are allowed. The restrictions focus on executable and script files that pose security risks.
Last modified on March 27, 2026
