Mail Drop in Apple Mail only works once, then large attachment e-mails disappear

Hello,

I’ve been having an unusual issue with Mail Drop in Apple Mail and I’m hoping someone might be able to help me troubleshoot.

I use both iCloud and Microsoft Exchange accounts in Apple Mail, and I also have an Apple One (iCloud+) subscription. Recently, I’ve been trying to send a large attachment via Mail Drop from my iCloud account.

Here’s what’s happening:

• If I send the large attachment once, it works fine — for example, I successfully sent it to my old Gmail address and to a family member.
• However, any subsequent e-mails with that same large attachment never go through. They disappear entirely — not in the Sent folder, not in Drafts, and not in Outbox.
• This happens even if I compose a new e-mail from scratch (not forwarding) and add the attachment again.
• Saving to Drafts before sending doesn’t resolve the issue.
• The problem seems specific to Mail Drop; if I send the same attachment through iCloud.com (web client), it works without issue.
• E-mails without large attachments send normally through Apple Mail with no problems.

It feels as though Mail Drop is “working” only for the first send, and then refuses to send the same attachment again.

Has anyone else experienced this, and is there a setting or known limitation that might explain it?

Thank you in advance for any suggestions.

__

PS: I'm running Sequoia 15.6 on a 15" MacBook Air (M4)