Maildoso SMTP mailboxes are designed for plain-text and light-HTML cold outreach. Attachments (PDFs, images, documents) are not delivered — the message itself goes out, but the attachment is removed, even for small files. This applies to campaign emails sent through a sequencer, to replies sent from the Master Inbox, and to messages sent from an email app connected over SMTP. It isn't something wrong with your account.
Why
Attachments are one of the strongest spam signals for a cold email from a new sender, and they can't be scanned and rotated safely through a sending pool. Sending attachments also slowed down delivery across the platform, so they're disabled for all Maildoso SMTP mailboxes. Keeping attachments out protects the reputation of your domains and of the pool everyone sends from.
What to use instead
Host the file (Google Drive, Dropbox, Notion, your website or a document-sharing tool) and send a plain link in the body of your email. For cold email this also performs better than an attachment.
With Global Custom Domain Tracking on, links are wrapped in a tracking domain consistent with your sending setup, which helps deliverability compared to a raw attachment.
If you need to send a file to a prospect who has already replied, send it from your regular company mailbox rather than from a Maildoso outreach mailbox.
Not sure whether this applies to your setup? Contact us with the mailbox address and how you're sending (sequencer, Master Inbox or mail app) and we'll confirm.