LMTP rejecting aliased emails

Hi, I am pretty new to Dovecot and totally new to LMTP. I have normal delivery working but I am struggling with aliases. In my mail queue I am seeing things like:

-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
E0F617055DB     720 Sat Oct 26 14:50:24 clamav@mydomain.com
(host mail.mydomain.com[private/dovecot-lmtp] said: 451 4.3.0
<clamav@mydomain.com> Temporary internal error (in reply to end of
DATA command))
clamav@mydomain.com

But in my /etc/aliases, I am aliasing clamav to nick and nick@mydomain.com is a valid mail box that can receive normal mail.

The corresponding line in the mail log is:

2024-10-26T14:55:56.385500+01:00 mail-www dovecot: lmtp(1371485):
Error: lmtp-server: conn unix:pid=1371484,uid=102 [1]: rcpt
clamav@mydomain.com: Failed to initialize user: Mail access for
users with UID 107 not permitted (see first_valid_uid in config
file, uid from userdb lookup).

How do I get round this issue? I can drop the minimum UID to 0, but then all system users are deemed valid for mail which is not true. None of them are.

Thanks,

Nick