With TLS1.2 the imaps server works correctly:
$ openssl s_client -connect mail.servicemouse.com:993
CONNECTED(00000003)
depth=2 C = US, O = Internet Security Research Group, CN = ISRG Root X1
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = R3
verify return:1
depth=0 CN = mail.servicemouse.com
verify return:1
---
Certificate chain
0 s:CN = mail.servicemouse.com
i:C = US, O = Let's Encrypt, CN = R3
1 s:C = US, O = Let's Encrypt, CN = R3
i:C = US, O = Internet Security Research Group, CN = ISRG Root X1
2 s:C = US, O = Internet Security Research Group, CN = ISRG Root X1
i:O = Digital Signature Trust Co., CN = DST Root CA X3
---
With tls1.0 there is no server certificate sent:
$ openssl s_client -tls1 -connect mail.servicemouse.com:993
CONNECTED(00000003)
write:errno=0
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 0 bytes and written 134 bytes
Verification: OK
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
Protocol : TLSv1
Cipher : 0000
Session-ID:
Session-ID-ctx:
Master-Key:
PSK identity: None
PSK identity hint: None
SRP username: None
Start Time: 1639736494
Timeout : 7200 (sec)
Verify return code: 0 (ok)
Extended master secret: no
---
Maybe the imap client is too old for that server configuration. You should revise the level of the TLS version backward compatibility that the mail server is supposed to provide.