You cannot start because the statements are not in the folder. The client thinks they already sent "everything." Your assistant drafted a patient explanation of why the year is complicated. They did not ask for a lecture. They asked, last month, what you still need.
This is the reverse of getting answers from your accountant. That article is the client writing to you with a theory. This one is you writing to them for paper.
Name the files as they would search
Respect the time they have. Open with the engagement and the date you actually need the folder complete. Then a short list, in their words: the December statements for the two accounts they already named, the 1099 they forwarded last year, the closing packet from the sale. Not "supporting schedules." People search for the words on the statement.
Say what you will do if an item does not exist — proceed without it, or hold that piece. A list with no "does not exist" option produces silence from the person who looked and failed.
Do not attach a checklist PDF. RelayLink does not carry attachments. Put the list in the briefing. Tell them where to put the files on the channel you already use. The reply here is "sent," "does not exist," or a question about one line.
Keep judgment out of the hunt
If you already have a reading of their year, leave it out of this message. A theory in a document request makes them defend a life instead of finding a PDF. The theory is a later briefing, after the folder is full, and it uses the discipline in the other article: facts first, then your reading, marked as yours.
Mark the premises about what they hold. You assumed the second entity still has its own account. You assumed the sale closed in the month they said. A labelled line gets a correction. A paragraph about "typical records" does not.
Follow up without growing the list
You approve the send. They do not need an account. They can reply from the email. If they already have an activated account, a new thread needs an accepted pair. A nickname you use for the entity stays on your side.
If they do not answer, follow up once with the same list and the same date. Do not add three more items to make the reminder worthwhile. That is how the original three die.
If two people share the records, one briefing still goes to one mailbox. Say who else has the login.
After they send
Write back which items you now have and which you still lack, in the same names. Ask them to correct that list. Then start the work that was blocked. Do not open a new theory in the thank-you note. That is a second ask, and it deserves its own message.
The return can still be hard. This email is only the folder.