Using the connection doctor I get the following logs, which makes this all even more strange. It seems that the mail.app client does not respond to the server's STARTTLS and pops an error about a minute later. I have no idea what this error means right now, but inquire minds want to know?!?!
LOGS:
Jul 1 15:09:59 client Mail[546] <Debug>: [0x600000361bc0] << 220 (89 additional bytes)
Jul 1 15:09:59 client Mail[546] <Debug>: Connected: <MFSMTPConnection: 0x600000361bc0> (Connected) account: A{SMTP - A356B9FF-6A70-4B81-9156-43BD847F0BED}
hostname: postoffice, port: 587, security layer: kCFStreamSocketSecurityLevelNone
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] >> EHLO (25 additional bytes)
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-postoffice Hello client [redacted], pleased to meet you
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-ENHANCEDSTATUSCODES
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-PIPELINING
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-8BITMIME
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-SIZE
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-DSN
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-AUTH GSSAPI
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-STARTTLS
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250-DELIVERBY
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 250 HELP
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] >> STARTTLS (0 additional bytes)
Jul 1 15:10:00 client Mail[546] <Debug>: [0x600000361bc0] << 220 2.0.0 (18 additional bytes)
Jul 1 15:11:00 client Mail[546] <Debug>: Could not connect: <MFSMTPConnection: 0x600000361bc0> (Disconnected) account: A{SMTP - A356B9FF-6A70-4B81-9156-43BD847F0BED} (Error Domain=NSPOSIXErrorDomain Code=60 "The operation couldn’t be completed. Operation timed out")