Do you see anything in the Delivery logs at the same time when sending to this user? 

Delivery log

13:25:33.386 [36438004] Delivery started for mail@domain.com at 1:25:33 PM
13:26:33.536 [36438004] Added to SpamCheckQueue (1 queued; 0/30 processing)
13:26:33.536 [36438004] [SpamCheckQueue] Begin Processing.
13:26:33.537 [36438004] Blocked Sender Checks started.
13:26:33.590 [36438004] Spam Checks started.
13:26:33.590 [36438004] Spam Checks skipped: User authenticated
13:26:33.590 [36438004] Spam Checks completed.
13:26:33.590 [36438004] Removed from SpamCheckQueue (0 queued or processing)
13:26:39.569 [36438004] Added to RemoteDeliveryQueue (1 queued; 0/50 processing)
13:26:39.569 [36438004] [RemoteDeliveryQueue] Begin Processing.
13:26:39.571 [36438004] Sending remote mail from mail@domain.com
13:26:39.573 [36438004] MXRecord count: '5' for domain 'gmail.com'
13:26:39.577 [36438004] Attempting MXRecord Host Name: 'gmail-smtp-in.l.google.com', preference '5', Ip Count: '1'
13:26:39.577 [36438004] Attempting to send to MXRecord 'gmail-smtp-in.l.google.com' ip: '142.251.116.27'
13:26:39.577 [36438004] Sending remote mail to: jackemailg@gmail.com
13:26:39.577 [36438004] Initiating connection to 142.251.116.27
13:26:39.577 [36438004] Connecting to 142.251.116.27:25 (Id: 1)
13:26:39.577 [36438004] Binding to local IP 104.IPSERVER.33 (Id: 1)
13:26:39.579 [36438004] Connection to 142.251.116.27:25 from 104.IPSERVER.33:50402 succeeded (Id: 1)
13:26:39.675 [36438004] RSP: 220 mx.google.com ESMTP 5614622812f47-3de22556d90si1035653b6e.60 - gsmtp
13:26:39.676 [36438004] CMD: EHLO mailout66.myserver.com
13:26:39.707 [36438004] RSP: 250-mx.google.com at your service, [104.IPSERVER.33]
13:26:39.707 [36438004] RSP: 250-SIZE 157286400
13:26:39.707 [36438004] RSP: 250-8BITMIME
13:26:39.707 [36438004] RSP: 250-STARTTLS
13:26:39.707 [36438004] RSP: 250-ENHANCEDSTATUSCODES
13:26:39.707 [36438004] RSP: 250-PIPELINING
13:26:39.707 [36438004] RSP: 250-CHUNKING
13:26:39.707 [36438004] RSP: 250 SMTPUTF8
13:26:39.707 [36438004] CMD: STARTTLS
13:26:39.738 [36438004] RSP: 220 2.0.0 Ready to start TLS
13:26:39.760 [36438004] CMD: EHLO mailout66.myserver.com
13:26:39.801 [36438004] RSP: 250-mx.google.com at your service, [104.IPSERVER.33]
13:26:39.801 [36438004] RSP: 250-SIZE 157286400
13:26:39.801 [36438004] RSP: 250-8BITMIME
13:26:39.801 [36438004] RSP: 250-ENHANCEDSTATUSCODES
13:26:39.801 [36438004] RSP: 250-PIPELINING
13:26:39.801 [36438004] RSP: 250-CHUNKING
13:26:39.801 [36438004] RSP: 250 SMTPUTF8
13:26:39.820 [36438004] CMD: MAIL FROM:<mail@domain.com> BODY=8BITMIME SIZE=4311
13:26:39.863 [36438004] RSP: 250 2.1.0 OK 5614622812f47-3de22556d90si1035653b6e.60 - gsmtp
13:26:39.863 [36438004] CMD: RCPT TO:<jackemailg@gmail.com>
13:26:40.082 [36438004] RSP: 250 2.1.5 OK 5614622812f47-3de22556d90si1035653b6e.60 - gsmtp
13:26:40.082 [36438004] CMD: DATA
13:26:40.113 [36438004] RSP: 354  Go ahead 5614622812f47-3de22556d90si1035653b6e.60 - gsmtp
13:26:40.425 [36438004] RSP: 250 2.0.0 OK  1724354800 5614622812f47-3de22556d90si1035653b6e.60 - gsmtp
13:26:40.425 [36438004] CMD: QUIT
13:26:40.456 [36438004] RSP: 221 2.0.0 closing connection 5614622812f47-3de22556d90si1035653b6e.60 - gsmtp
13:26:40.457 [36438004] Attempt to ip, '142.251.116.27' success: 'True'
13:26:40.459 [36438004] Process delivery status notification step from recipient success. Recipient: [jackemailg@gmail.com], Notify: [], LastError: [], RanDomainFilter: [False], RanGlobalFilter: False
13:26:40.459 [36438004] Delivery for mail@domain.com to jackemailg@gmail.com has completed (Delivered)
13:26:40.459 [36438004] Removed from RemoteDeliveryQueue (0 queued or processing)
13:26:42.567 [36438004] Removing Spool message: Killed: False, Failed: False, Finished: True
13:26:42.567 [36438004] Delivery finished for mail@domain.com at 1:26:42 PM    [id:36438004]

08:48:12.014 TlRMTVNTUAADAAAAGA.........AYwBsAAAAAAAAAAAA Object reference not set to an instance of an object.
   at SmarterMail.Protocols.Authentication.AuthenticateMessage.TryParse(Byte[] data, String hostname, String& failure, List`1& logLines, Boolean removeFromCache)
   at SmarterMail.Protocols.Authentication.NegotiateMessages.Parse(Byte[] data, String& response, String base64, String ip, String hostname, ProtocolTypes types, Boolean removeFromCache, String userAgent)
   at SmarterMail.Protocols.Authentication.AuthenticationManager.Parse(String base64, String ip, String hostname, ProtocolTypes types, Boolean removeFromCache, String userAgent)
08:48:43.725 TlRMTVNTUAADAAAAGA.......QByAC4AYwBsAAAAAAAAAAAA Object reference not set to an instance of an object.
   at SmarterMail.Protocols.Authentication.AuthenticateMessage.TryParse(Byte[] data, String hostname, String& failure, List`1& logLines, Boolean removeFromCache)
   at SmarterMail.Protocols.Authentication.NegotiateMessages.Parse(Byte[] data, String& response, String base64, String ip, String hostname, ProtocolTypes types, Boolean removeFromCache, String userAgent)
   at SmarterMail.Protocols.Authentication.AuthenticationManager.Parse(String base64, String ip, String hostname, ProtocolTypes types, Boolean removeFromCache, String userAgent)

08:48:44.984 [11.11.11.11] SMTP Login successful: For user admin@domain.com 
08:49:12.860 TlRMTVNTUAADAAAA........wBsAAAAAAAAAAAA Object reference not set to an instance of an object.
   at SmarterMail.Protocols.Authentication.AuthenticateMessage.TryParse(Byte[] data, String hostname, String& failure, List`1& logLines, Boolean removeFromCache)
   at SmarterMail.Protocols.Authentication.NegotiateMessages.Parse(Byte[] data, String& response, String base64, String ip, String hostname, ProtocolTypes types, Boolean removeFromCache, String userAgent)
   at SmarterMail.Protocols.Authentication.AuthenticationManager.Parse(String base64, String ip, String hostname, ProtocolTypes types, Boolean removeFromCache, String userAgent)

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08:48:07.750 [13829001] Delivery started for admin@domain.com at 8:48:07 AM
08:48:46.889 [13829002] Delivery started for admin@domain.com at 8:48:46 AM
08:49:07.925 [13829001] Added to SpamCheckQueue (1 queued; 0/30 processing)
08:49:07.926 [13829001] [SpamCheckQueue] Begin Processing.
08:49:07.929 [13829001] Blocked Sender Checks started.
08:49:07.930 [13829001] Blocked Sender Checks completed.
08:49:08.023 [13829001] Spam Checks started.
08:49:08.023 [13829001] Spam Checks skipped: User authenticated
08:49:08.023 [13829001] Spam Checks completed.
08:49:08.174 [13829001] Removed from SpamCheckQueue (0 queued or processing)
08:49:10.930 [13829001] Added to LocalDeliveryQueue (1 queued; 0/50 processing)
08:49:10.930 [13829001] [LocalDeliveryQueue] Begin Processing.
08:49:11.005 [13829001] Starting local delivery to admin@domain.com
08:49:11.144 [13829001] Process delivery status notification step from local recipient success. Recipient: [admin@domain.com], Notify: [], Delivered: [True], Forwarded: [False], Deleted: False
08:49:11.144 [13829001] Delivery for admin@domain.com to admin@domain.com has completed (Delivered) Filter: None
08:49:11.144 [13829001] End delivery to admin@domain.com (MessageID: <52b7079f94254009821036f35abb5919@8fdab062631045278e0999c5aa59d66d>)
08:49:11.145 [13829001] Removed from LocalDeliveryQueue (0 queued or processing)
08:49:13.940 [13829001] Removing Spool message: Killed: False, Failed: False, Finished: True
08:49:13.940 [13829001] Delivery finished for admin@domain.com at 8:49:13 AM    [id:611913829001]

Hello Jack, I created a ticket for this issue for you so that we can dive in to this. I will reply to it shortly.