We are currently experiencing a bug in altmail whereby if there is a significant delay between adding an attachment and sending (say 3+ minutes) altmail can sometimes fail to send the attachment.
In order to avoid this issue we recommend using https://ox.blacknight.com instead.
You may need to enable it for your email accounts: https://help.blacknight.com/hc/en-us/articles/115000598125-OX-Mail-Assigning-OX-Mail-to-new-and-existing-accounts-
Alternatively you can set the account up in a mail client like Outlook which won't have this issue, as per:
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