You're right, double checked and it always defaults to smtp. instead of .my

When manually changing on both PC and Mobile, the SMTP accounts can be added without the certificate error!

Thanks Everyone!

It seems to be only with Outlook, as when connecting via Thunderbird the certificate alert isn't prompted.

the certs look correct and valid . So, it's some outlook client issue . I don't have a Windows PC to test but by mobile do you mean Android app ?

These are the screenshots of the certificate...

It seems to be only with Outlook, as when connecting via Thunderbird the certificate alert isn't prompted.

The email service has been restarted a few times, with no change in the alert.

When running the openssl s_client -connect my_imap_server:993 -showcerts command with my domain i get the below...

openssl s_client -connect my.promentum.info:993 -showcerts
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 = R10
verify return:1
depth=0 CN = my.promentum.info
verify return:1
---
Certificate chain
 0 s:CN = my.promentum.info
   i:C = US, O = Let's Encrypt, CN = R10
   a:PKEY: rsaEncryption, 4096 (bit); sigalg: RSA-SHA256
   v:NotBefore: Apr 24 06:12:01 2025 GMT; NotAfter: Jul 23 06:12:00 2025 GMT
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
 1 s:C = US, O = Let's Encrypt, CN = R10
   i:C = US, O = Internet Security Research Group, CN = ISRG Root X1
   a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA256
   v:NotBefore: Mar 13 00:00:00 2024 GMT; NotAfter: Mar 12 23:59:59 2027 GMT
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
---
Server certificate
subject=CN = my.promentum.info
issuer=C = US, O = Let's Encrypt, CN = R10
---
No client certificate CA names sent
Peer signing digest: SHA256
Peer signature type: RSA-PSS
Server Temp Key: X25519, 253 bits
---
SSL handshake has read 3685 bytes and written 399 bytes
Verification: OK
---
New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384
Server public key is 4096 bit
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)
---
---
Post-Handshake New Session Ticket arrived:
SSL-Session:
    Protocol  : TLSv1.3
    Cipher    : TLS_AES_256_GCM_SHA384
    Session-ID: 4BD471C720F74F914D6FD783B8289F6CA08BA52BFAF89D2D46B8D89F992845D7
    Session-ID-ctx:
    Resumption PSK: 1B942C5384C631FA71F4D00D41838CCC0F1A02A91563D3085A555F10BDBBCB8B049495AC765610B1B711BD5728F188BA
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    TLS session ticket lifetime hint: 7200 (seconds)
    TLS session ticket:
    0000 - a3 8b 31 e5 cf 07 1e 35-20 f2 7d ce 82 ee 02 61   ..1....5 .}....a
    0010 - ea ff f1 f0 17 1e 74 0c-79 48 a3 6b b3 2d 2e 69   ......t.yH.k.-.i
    0020 - fb 3f 92 1a 2c 81 74 10-17 11 70 32 74 99 35 12   .?..,.t...p2t.5.
    0030 - 01 b0 78 e2 c3 3a 9d ed-3a bc 7b 2c dc 89 be 81   ..x..:..:.{,....
    0040 - c5 0c 3b 63 bb 9c e1 b9-54 5c b4 94 4b 97 c5 77   ..;c....T\..K..w
    0050 - 82 85 7d 42 1b d3 10 6c-e9 01 30 95 13 2e de 5c   ..}B...l..0....\
    0060 - 6f 05 dc 5f 45 11 51 d0-01 08 78 c7 71 76 09 f6   o.._E.Q...x.qv..
    0070 - ec df b6 86 2e fb 13 b2-bf 68 a8 33 80 72 4d b2   .........h.3.rM.
    0080 - 0b b6 ab f2 f2 30 8b 64-f3 bf 60 18 bb cb 6d 2e   .....0.d..`...m.
    0090 - e7 6a 8e a0 d4 06 2d 28-8a ef e4 43 ff bd ca d9   .j....-(...C....
    00a0 - ca 8c 87 e2 97 c5 e2 c8-90 ac 16 e9 ff f5 63 31   ..............c1
    00b0 - 08 87 d3 76 c8 a7 f8 3e-a4 6f 95 17 bc 36 a6 a1   ...v...>.o...6..
    00c0 - 37 53 8b da ba 50 35 ad-50 3d 34 b6 77 9c cd 3b   7S...P5.P=4.w..;
    00d0 - 36 02 5c 48 93 dd ba 45-bb 2d ce b6 63 59 b6 58   6.\H...E.-..cY.X

    Start Time: 1746784945
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)
    Extended master secret: no
    Max Early Data: 0
---
read R BLOCK
---
Post-Handshake New Session Ticket arrived:
SSL-Session:
    Protocol  : TLSv1.3
    Cipher    : TLS_AES_256_GCM_SHA384
    Session-ID: 85CF897B8F0BB087A3ADDD697F374EF68FB6A7E0B5CD8A6ABA6D0D71B882B7DF
    Session-ID-ctx:
    Resumption PSK: AE9A476653A062F858587ED044F35B7A574AF3DFEAE1665F4025F62FE3FB4F76A66F501F212FFC0534D90E6BDF6A5BFB
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    TLS session ticket lifetime hint: 7200 (seconds)
    TLS session ticket:
    0000 - a3 8b 31 e5 cf 07 1e 35-20 f2 7d ce 82 ee 02 61   ..1....5 .}....a
    0010 - 75 9a 28 8b 45 5b d4 c8-b7 a6 48 e7 6a ab a2 ec   u.(.E[....H.j...
    0020 - 7f b1 83 33 3e d1 db 4b-db 68 ec 05 58 6e 29 c7   ...3>..K.h..Xn).
    0030 - 98 6c a5 50 61 2a d9 10-89 bd 8a d5 99 a6 77 f6   .l.Pa*........w.
    0040 - dd 59 b0 0f 51 70 75 bf-54 2c 46 86 71 d3 63 cb   .Y..Qpu.T,F.q.c.
    0050 - 93 22 f8 0f b9 73 1b 98-12 45 3b 1a 3b 0c f7 93   ."...s...E;.;...
    0060 - f2 b3 b5 74 11 90 0b 6d-c9 ec 0c cc 8e 8e 24 a5   ...t...m......$.
    0070 - 14 61 a8 d1 39 99 14 23-2e 7f 23 c5 61 b0 07 f9   .a..9..#..#.a...
    0080 - a4 31 22 a3 1d 2f 78 51-f4 54 19 8c 4f 82 73 e7   .1"../xQ.T..O.s.
    0090 - 0b f0 18 fe 55 6f e5 de-ea 49 b5 95 b2 17 d9 d4   ....Uo...I......
    00a0 - 9f a1 0f 18 5f 45 42 32-72 43 42 e6 bb 6e d3 d4   ...._EB2rCB..n..
    00b0 - bb 77 5a 0a 9f 0b 03 a1-06 7a 9e 95 3e da 8a 90   .wZ......z..>...
    00c0 - 5a 53 a0 9b 1f 0c 8d 4e-99 25 ed 87 64 28 81 f4   ZS.....N.%..d(..
    00d0 - af 83 b8 2b cd a5 8d 87-99 56 f3 43 e2 87 52 32   ...+.....V.C..R2

    Start Time: 1746784945
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)
    Extended master secret: no
    Max Early Data: 0
---
read R BLOCK
* OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE LITERAL+ AUTH=PLAIN] Dovecot (Ubuntu) ready.

You can also inspect the cert with openssl s_client -connect my_imap_server:993 -showcerts