Personal letters, no matter the volume.
Generate a letter from an HTML template or upload a PDF. MailStream prints it, folds it into a white #10 double-window envelope, and mails it within 1 to 2 business days with full delivery tracking.
Formats
One sheet size. One envelope. One API.
Every letter prints on 8.5×11 sheets and mails in a white #10 double-window envelope. Your logo and return address show through the top window, the recipient address and barcode through the bottom, so there is nothing to print on the envelope itself.
Single-page letters
One 8.5×11" sheet, printed one- or two-sided, folded in thirds into a #10 double-window envelope. Ideal for invoices, reminders, and one-page correspondence.
Templates
Start from our artwork template.
Download the 8.5×11 letter template and drop your design in. It marks the address block MailStream prints over your artwork, the two window areas, the fold lines, and the safe zone.
- HTML starter templateHTML · 4 KB
Merge variables, page breaks, and the address block already positioned. Edit the copy and send.
- Print templatePDF · 97 KB
Exact dimensions, window positions, fold lines, and safe zone for your designer or print workflow.
- Photoshop templatePSD · 4.2 MB
Layered file with guides for the address block, window areas, and safe zone.
- All artwork templatesZIP · 8.6 MB
The letter template plus 4×6, 6×9, and 6×11 postcards in AI, PSD, and PDF.
Capabilities
Built for transactional and marketing mail alike.
- HTML or PDF input
- Design your letter in HTML with merge variables, or upload a print-ready PDF. Start from our 8.5×11 template so your artwork clears the address windows.
- Merge variables
- Personalize every letter with recipient-specific data: names, account numbers, balances, dates, or any custom field your workflow requires.
- Batch sending
- Upload a mailing list and let Mailstream produce every letter in a single run. Thousands of letters, each uniquely personalized, processed within 1 to 2 business days.
- Mail barcode tracking
- Every envelope is tracked from production through USPS handoff to delivery. Monitor status via the dashboard, API, or real-time webhooks.
- Address standardization
- Addresses are CASS-verified and standardized before printing. Bad addresses are flagged for correction before postage.
- Proof preview
- Review a rendered PDF proof of your letter before it prints. Approve individually or enable auto-approval for high-volume, template-driven sends.
Input
Personalization
Scale
Tracking
Deliverability
Quality
- 1–2
- Business-day turnaround
- 6
- Max sheets per envelope
- #10
- White double-window envelope
- 100%
- Delivery tracking
- We are the manufacturer
- No middlemen or delays. Your mail goes directly from our floor to USPS, with quality checks.
- HITRUST-certified facility
- Your data runs in a HITRUST-certified environment with enterprise security and compliance.
- 25+ years of expertise
- Twenty-five years in direct mail, starting with an early online printer in 1998. We’ve tackled every edge case.
- Real-time USPS tracking
- Track Intelligent Mail barcodes. Know when printed, in transit, and delivered via dashboard, API, or webhooks.
Use cases
Physical letters for every business need.
- Invoices & statements
- Automate monthly statements and invoices for customers who require or prefer physical copies. Trigger sends from your billing system via API.
- Compliance & regulatory notices
- Healthcare EOBs, insurance notices, financial disclosures. When regulators require physical delivery, Mailstream handles the logistics.
- Welcome letters
- Welcome new customers with a personalized letter and their account details, triggered the moment they sign up. First impressions matter. Make yours tangible.
- Fundraising appeals
- Nonprofits see 3-5x higher response rates from direct mail than digital channels. Send personalized appeals segmented by giving history, straight from your donor CRM.
Payload format
Predictable, typed JSON payloads.
Every webhook payload follows the same envelope structure. The nested data object contains the full mail piece resource at the time of the event.
What everyone is saying
Trusted by professionals.
We replaced three vendors with Mailstream and finally got full visibility from print to delivery. Tracking every piece in real time changed how we measure direct mail.
Daniel H.
VP of Growth, Northbridge Labs
The HTML design tool is a game changer. We design, personalize, and send without ever leaving the platform. No more emailing PDF proofs back and forth.
Olivia W.
Head of Lifecycle, Brightside
We integrated the API in a day and had automated compliance notices running by the end of the week. The HITRUST certification made our security team very happy.
Ethan K.
Engineering Lead, Cornerstone Health
Join agencies, SaaS teams, and operators using MailStream to automate direct mail at scale without manual vendors or guesswork.